Many of the USB wifi 'dongles' around use the ZD1211 chipset. This is supported reasonably well in Linux, although not in 'master' mode, meaning your Bifferboard will only be able to communicate with a base station, rather than become one. If that limitation is acceptable, then read on.
The link above points to the ZD1211RW community driver which is present since kernel 2.6.18.
The following USB WLAN sticks using this driver are working fine on Slackware 2.6.32 on the bifferboard:
- CC&C's WL-2203S-V3, found cheap on eBay (with external antenna)
- icidu NI-707503 (identical to WLG-1500SMA found on eBay) - internal antenna
- MSI US54SE - internal antenna
There is one caveat; a number of people have experienced that when the MTU is larger than 500 this driver on slackware on the BB aborts telnet/ssh sessions when a larger amount of data is sent (for instance doing an ls /dev). With an MTU of for instance 448 (ifconfig wlan0 mtu 448) the devices run fine albeit slower (speed seems to be halved).
The Zydas vendor driver (22.214.171.124) available here:
with this patchset:
Also a Thomson ST121g (Prism54 chipset) works fine (with any MTU) as well as an Asus WL-160N (Ralink RT2870 based).
Using a wireless dongle on the 2xUSB bifferboard is convenient when an SD card is used for rootfs/storage. The dongle then fits directly into the USB port. Using the USB port for rootfs/storage will require a cable for the wlan dongle since these dongles are too wide, blocking the other USB port.
There are nowadays poststamp sized mini wlan dongles using for instance Realtek rtl8192su chips (2T2R) that do fit next to an USB port.
I bought one of these http://www.ccandc.com.tw/products_2.php?prod=35 from eBay which is rtl8188su based (same as rtl8192su but 1T1R) and it works fine using the (2.6.32) staging driver for rtl8188su/8191su/8192su. Had to edit the file "/drivers/staging/rtl8192su/r8192U_core.c" to include the 0x0bda:0x8171 VID/PID pair.
To summarize (more info at http://www.realtek.com.tw/products/productsView.aspx?Langid=1&PNid=21&PFid=48&Level=5&Conn=4):
Update: I also bought this one: http://www.xtreamer.net/accessories/wifi-usb-antenna.aspx which is rtl8191su based (802.11n 1T2R) which works fine with the staging rtl8192 driver. The benefits of this adapter are that the second USB adapter of the 2xUSB BB remains accessible and that it has an external adjustable antenna.
Edit: I also tried a Ralink RT3070 based device with the vendor driver, version 126.96.36.199 did not compile but version 188.8.131.52 works fine. This single-chip device is also used in some tiny USB WiFi devices.