pfSense includes built in wireless capabilities that turn a pfSense firewall into a wireless access point, use a wireless 802.11 connection as a WAN connection, or both.
There are a variety of wireless cards supported in FreeBSD, and pfSense includes support for every card supported by FreeBSD. Some are supported better than others. Most pfSense developers work with Atheros hardware, so it tends to be the most recommended hardware. Many have success with other cards as well.
Linksys, D-Link, Netgear and other major manufacturers commonly change the chipsets used in their wireless cards without changing the model number. There is no way to ensure a specific model card from these vendors will be compatible because there is rarely a way of knowing which "minor" card revision is being purchased. While one revision of a particular model may be compatible and work well, another card of the same model may be incompatible. For this reason, we recommend avoiding cards from the major manufacturers. If one is already on hand, it's worth trying to see if it is compatible, but be warned if purchasing one because the "same" model worked for someone else, it may end up being a completely different piece of hardware that is incompatible.
The spreadsheet linked below contains a compatibility matrix and driver list for cards that should work with pfSense 2.0 and later - be aware of the hostap column, which indicates which drivers are capable of running in access point mode. If the hostap column is marked N, then the card may only be used as a client.
The second tab on the sheet lists part numbers for a given driver.
This information was compiled from FreeBSD man pages and driver source code, and some from submitted information/experience.