If you're looking for a modern laptop or workstation to run NetBSD onto, you're in the right place.

Useful resources

Before posting here, please make sure to check these sources (both official and unofficial)

man page section 4 - Special Files and Hardware support. Start with pci(4) and usb(4) for an up-to-date list of available drivers and related supported devices.
BSD Hardware Database - Computers powered by NetBSD
NYC*BUG dmesgd. Fully searchable *BSD dmesgs; very useful.
These threads on r/NetBSD: [1] [2]
These threads on Daemon Forums: [1][2]
netbsd-users mailing list, marc.info allows to search for keywords through the mailing list archive, making it an unparalleled source of information. If you' don't find what you're looking for, then please subscribe and post: the list is very active and is representative of the netbsd community as a whole.
#netbsd channel on Libera Chat; it's quite active and, you'll likely get a reply in no time.

General hardware support considerations

These notes reflect NetBSD hardware compatibility as found in 10.0 release.
Focus is on x86_64 hardware.

SSDs:
- SATA III / M.2 SATA models supported via atabus(4)
- PCIe (NVMe) models suppported via nvme(4)
Graphics:
- AMD Radeon up to Polaris (amdgpu(4)); RX 570 was reported working here, but RX 590 should work as well.
- Nvidia up to Pascal (nouveau(4)); my GTX 1060 works fine; GTX 1070 was reported working.
- Intel embedded graphics (i915drm(4)). SkyLake works well; I think Kaby Lake is supported too. Anything beyond probably not.
- avoid hybrid nvidia/ intel graphics at all costs.
Ethernet : Intel wm(4), Realtek re(4), and Broadcom bge(4) NICs are well supported. I can't recommend enough my PCIe BCM5720 Broadcom NeXtreme Gigabit Ethernet adapter.
Wifi: Intel cards (Wifi Link, Centrino, Wireless AC) probably have best support. Look up iwm(4) and iwn(4) to get a list of supported chipsets. Possiblle alternatives: bwfm(4), athn(4); for USB dongles, see: urtwn(4) and run(4). Don't expect 802.11ac/ax (at least not yet, hopefully).
A renewal of the wifi stack is in the works and some drivers (namely intel and realtek ones) have already been converted.
Suspending . Very unpredictable. Nvidia GPUs are known to prevent resuming as do some Intel wireless cards (Note: my workstation equipped with a Nvidia GTX 1060 actually does resume). Thinkpads generally have better ACPI support, with the addition of working multimedia and brightness function keys, thanks to a dedicated thinkpad(4) driver. Here are some Supported Thinkpads. Likewise, fujitsu laptops should have working brightness control, pointer, and hotkeys, using the fujbp(4) driver.
Audio: supported by hdaudio(4) in the overwhelming majority of cases. Some modern discrete Sound Blaster cards (Rx seriers with E-MU schips) may lack a driver (FreeBSD supports them). Sound Blaster Audigy Fx instead is supported by hadaudio(4) (tested and working), and Sound Blaster PCI 512 is supported by emuxki(4).
Webcams (USB bus); anything I tested was supported by uvideo(4) / uaudio(4)
Bluetooth: NetBSD has good bluetooth support, for USB and UART transport layers. See ubt(4) and btuart(4).
Misc: No problem with XHCI, USB and PS/2 input devices, USB serial adapters. For SSD/MMC card readers better find a Realtek model, most likely supported by rtsx(4).

Suggested hardware (x86_64)

1. Workstations

Buy a used or refurbished professional workstation (2018 or older, for acceptable hardware compatibility):

You can also build up your own workstation, buying components (can get used ones in great shape on eBay). Or buy a used workstation and upgrade parts to maximize specs.

2. Laptops

See the 'Useful Resources' list above, as most third-party links focus on laptop support. Again, don't expect to run NetBSD on very recently marketed models: most likely graphics aren't going to be supported. (Why? See [1]).
You may have better luck with business-grade UNIX-friendly lines, including Lenovo Thinkpad, HP ZBook and Dell Latitude. These are often acquired by resellers from companies in the process of renewing their hardware, and may be found in very good shape at an affordable price.

Don't be afraid to replace unsupported parts. I've turned unsupported laptops into compatible ones by replacing the wifi chips for 20$ or so. Also, keep in mind that RAM is almost always expandable (and you'll probably need it if you plan on developing and building from source).

[1] Keeping the drm/kms stack in sync with Linux takes a lot of effort and the project only has a very limited number of developers to count upon, with a wide range of architectures and systems to take care of; it would be unreasonable to ask them to dedicate all their time to the drm stack.

Non x86_64 architectures

1. evbarm64 (aarch64)

PINEBOOK Pro. NetBSD has been supporting Pinebook Pro since the early days of its release thanks to the work carried out by @ebijun. Subscribe to the netbsd-port-arm mailing list, to get periodical updates on the Pinebook port.
ROCKPro64. very well supported (including accelerated graphics) thanks to the effort of @jmcneill
Raspberry Pi 4 Model B. well supported (but GPU acceleration; a partial workaround is represented by multimedia/omxplayer which attaches directly to the GPU). Considering how Raspberry pioneered the market of ARM SBCs, and how broadly available are they, it is reasonable to assume that NetBSD will continue to support Raspberry boards in future.
Other ARM (64) microcontrollers: NetBSD supports a large variety of ARM boards (including BeagleBone Black and Hardkernel Odroid C2), see NetBSD Arm Bootable Images.
Apple Mac mini (M1, 2020). NetBSD 10 only has initial support for Apple Silicon (working), but again, it's likely going to improve in the foreseeable future.
At the moment of writing, OpenBSD may have better support for Apple M1/M2, see OpenBSD/arm64

2. sparc64

The NetBSD sparc64 port is a Tier1platform (much like amd64 and aarch64); binary packages are constantly published and support is generally very good.

Note: support for Oracle UltraSPARC T2-T5, and Fujitsu SPARC64 is still a working in progress. I'm not aware of anybody working on a Oracle SPARC M5-M7 port.
If you own any of these models, keep in mind that OpenBSD already supports them, and in case refer to OpenBSD/sparc64.

Recommended hardware

Servers: SunFire V480 (or even SunFire V880). These are fabulous machines and can still be competitive by modern standards. If you want to run NetBSD/sparc64 on something which doesn't necessarily qualifies as retrocomputing, get a SPARC v9 / UltraSPARC III computer. The advantage is that, since we're not speaking of classic SPARC stations, they don't attract enthusiasts that much, and can paradoxically be found at quite affordable prices.
Cheap alternative: I have a sweet spot for the SUN Netra line of rackable servers, in particular the Netra X1 and Netra T1.
Cheaper alternative (~ 100-150€): SunFire V125. These were intensely marketed by Oracle, so it's easy to find one.
Workstations: If you really want it, get a Sun Blade, like Sun Blade 1500. These actually attract enthusiasts and are going to cost quite a bit.

3. alpha

NetBSD was the first opensource OS to introduce a port for DEC Alpha in the distant 1995. Support has been historically very solid (and powered SDF.org for decades). The history of the Alpha architecture is, in retrospect, very interesting, so I'll link this thread on Retrocomputing StackExchange as a digression.

If you want to get your hands on a Alpha computer but would rather have something which doesn't occupy too much room and won't cost you a fortune, you can get a Compaq Alpha Server DS10.
To use NetBSD on AlphaServer DS25, refer to these threads on port-alpha: [1] [2]

4. macppc

NetBSD is probably, alongside OpenBSD, the only real alternative to early OS X on Apple PowerPC.
Recommended hardware:

Apple Power Macintosh G5 Dual Core (970MP). This is probably the best you can get out of the PowerPC Apple line, and is still quite affordable (pretty much like all PowerPC Apple hardware).
Apple Mac mini G4/1.42 (7447A). This a nice to have, and is ideal to play with NetBSD on something which isn't amd64 or aarch64.

5. i386

Pentium 4 and Pentium M may be the only processors to still be capable of running a modern gecko-based browser (Seamonkey, Arcticfox) reasonably well.
While dabbling with NetBSD on 486 will be a joy for the heart of the computing nerd, if you want to work on the i386 port, or simply attempt to use a x86 machine as an alternative to your daily driver, I'll recommend looking for Pentium M powered laptops (usually equipped with WindowsXP):

Still want to run NetBSD on 486?
You can get an embedded box-PC or SoC equipped with a IA-32 compatible Vortex86 CPU, at least the models implementing a FPU, like the Vortex86 DX: