OpenWrt is a niche Linux distribution, which enables you to deploy a vast number of software. Your hardware is the only limit. This guide is intended to help you pick the right hardware for the intended utilization.

OpenWrt does not recommend any hardware or manufacturer! There is no "best hardware", so stop asking. Purchase something that meets your requirements. Inform yourself about the current hardware support in the Internet and ask other users/developers for a personal recommendation in the forum. A personal recommendation can be informative and helpful, or it can simply be a pile of crap. Unproven claims and make-believes. Avoid overhyped overpriced shit. Embedded hardware can be VERY low cost! This is something good! The magic is done by OpenWrt.

• Bootloader
  • the bootloader should not only be under any FOSS license, but preferably under the GPL, so that they really are forced to release the complete source code
  • some bootloaders make installing OpenWrt unnecessarily complicated or even impossible!
  • some bootloaders allow you to boot from a USB device or Boot over Ethernet but many do not
• LAN: / Ethernet
  • How many NICs? Common are one or two (Ethernet MAC-blocks integrated into the SoC, which are connected over a xMII to a PHY (chip) on the PCB.)
  • Which Layer 1 standard shall the NIC/NICs support? BASE100-TX (100MBit/s, aka Fast Ethernet) or BASE1000-T (1000MBit/s, aka Gigabit Ethernet)?
  • Is there an integrated ethernet switch?
  • Which Layer 1 standard does the integrated switch support?
  • How many ports does it have?
  • Is the switch manageable? Which capabilities does it offer? 4bit VIDs, 4bit VIDs,
• WLAN / Wireless: (Please consult wireless.overview)
  • how many WNICs? Common are one or two integrated into the SoC or up to four MiniPCI or MiniPCIe sockets.
  • Which substandards of the IEEE 802.11-family shall the wireless hardware support? Most common ones are IEEE 802.11a, IEEE 802.11b, IEEE 802.11g, IEEE 802.11n and IEEE 802.11s.
  • Frequencies (or bands):
    • 2.4 GHz or 5 GHz or both? (see e.g. em interference at 2.4GHz or Fresnel zone)
    • for the AP to be capable to run in 2.4 GHz and 5 GHz at the same time, the router must support dual band simultan aka DBDC (DualBand-DualConcurrent). This tag shall help you find suitable devices more quickly:

      802.11abgn simultan
  • At 2,4GHz you only have 3 distinct channels without overlap, in the 5GHz band there are 19 (EU)/ 13 (USA)/ ?? (Japan) channels without overlapping available
  • the 2,4GHz band is quite crowded by your neighbours, by a Bluethooth PAN, etc. while the 5GHz is usually unused
  • Is it relevant to you, whether the WNICs are SoftMAC or FullMAC devices?
  • Do the current capabilities of the Linux FOSS drivers for your WNICs satisfy your requirements?
  • are the antennae detachable? If so, you could replace them with ones with a better gain, or with (self made) directional antennae
• Modem:
  • Common Layer 1 protocols that utilize a Modem are: DSL, DOCSIS and Mobile telephony, e.g. LTE.

    ADSL2plus, VDSL2, DOCSIS, Mobile telephony
  • If a device has a built-in modem, is the modem fully supported by OpenWrt?
  • E.g. there are no FOSS drivers for the Broadcom modems in the BCM63xx SoCs, but there are FOSS drivers for Texas Instruments/Infineon/Lantiq Modems in these SoCs: soc.lantiq!
• Bottlenecks: RAM, CPU, Flash
  • the most crucial bottleneck is the RAM. If you are going to run asterisk, mumble, Direct Connect, BitTorrent, a web server and other stuff, enough RAM will make them run smoothly. Some of them tolerate SWAP pretty good, others do not. FYI: If you already consider soldering (much) more RAM, keep in mind that there no DDR1-Modules bigger then 64MiB. Also, the SoCs sometimes only support so much. For example, the Marvell Kirkwood, supports only a maximum of 512MiB.
  • on seldom occasions the computing power of the CPU proves to be a bottleneck. To compare you should have a look at the whole SoC, which contains the CPU. Do not compare raw MHz, e.g. a MIPS 34KE@300MHz is in most scenarios faster then a MIPS 24K@400MHz.
  • as you can see here with a total of 8MiB flash memory, you can use about 5MiB for own packages.
• Ports:
  • USB: connect a hub, harddiscs, ssds, usb sticks, UMTS modems, cameras, sound cards, etc. →usb.overview
  • Serial: very useful for developers, limited uses for end users →port.serial
  • JTAG: very useful for developers and also for end users →port.JTAG
• Buttons:
  • buttons: with a button you can boot your device into OpenWrt Failsafe, without one, this is not possible! Also, after boot up, you can attach a function to it, like start/stop WLAN, reconnect DSL, start/stop a daemon, mount/unmount partitions, etc.
• Other, e.g.
  • the Redwave RW2458N is small device and offers a MiniPCIe slot
  • the ALFA Network Hornet-UB is small device and supports 12V Passive PoE (Power over Ethernet)
  • the ASUS WL-330N is also just small

Since Christmas 2011, geizhals offers a filter for devices supported by OpenWrt:

• Preisvergleich AT: WLAN-Router
• Preisvergleich DE: WLAN-Router
• Porównanie cen PL: WLAN-Router
• Price comparison UK: WLAN-Router

• Preisvergleich AT: WLAN-Router mit Modem
• Preisvergleich DE: WLAN-Router mit Modem
• Porównanie cen PL: Routery WLAN z modemem
• Price comparison UK: WLAN Router with Modem

Note_1: You can safely ignore the filters for "3G-Router" and "BitTorrent-Client", since they apply to the OEM firmware only. Once you installed OpenWrt, the full software repository stands to your disposal.
Note_2: Do not expect the filter to be perfectly up-to-date, since it is updated manually conforming to the OpenWrt ToH!

Maybe because of the matured FOSS wireless drivers, maybe for completely different reasons, some devices are much more popular than others. There is more outlay concerning them in the Forum and, since they have more users, their Wiki-Pages are a bit more exhaustive.

You could use the tags to quickly find the devices with your desired features. Sadly not many devices have been tagged so far.