UPDATE (2013/09/02): I found that the wireless adapters were no longer able to run in Ad-Hoc mode due to recent changes to support the Beaglebone Black device. The installation steps on the Github project page have been updated to specify a commit tag (v0.2.0) when checking out the HSMM-Pi project to ensure you’re getting a stable copy of the code.
My initial recommendation for USB WiFi adapters was an inexpensive model that could be found on Amazon for about $7. They worked fairly well initially, but in the last week I’ve been unable to get the 3 that I own to work at all. I am no longer recommending them for use with Raspberry Pi models because of their instability.
I purchased the USB WiFi adapter sold by Adafruit which is advertised as being compatible with the Raspberry Pi. These adapters work flawlessly, and use a different chipset from the model I recommended earlier. The Adafruit adapter costs about $11 and does not accommodate an external antenna; but these negatives are offset by the adapter being stable.
My apologies to anyone who has had problems with the Amazon no-name adapter. And it would be great to hear from anyone who has had problems but managed to get the adapters to be stable.