$ apropos wireless shows me a list of 28 WiFi drivers, 11 of them with USB attachment in the man page description. athn(4) has USB connectivity but doesn't mention it there; and there may be others among the 28 like that.
Here are the 11 that are specifically USB attachable. I have a urtwn(4) device, for use as a backup.
Code:
atu(4) Atmel AT76C50x USB IEEE 802.11b wireless network device
otus(4) Atheros USB IEEE 802.11a/b/g/n wireless network device
rsu(4) Realtek RTL8188SU/RTL8192SU USB IEEE 802.11b/g/n wireless network device
rum(4) Ralink Technology USB IEEE 802.11a/b/g wireless network device
run(4) Ralink Technology USB IEEE 802.11a/b/g/n wireless network device
uath(4) Atheros USB IEEE 802.11a/b/g wireless network device
upgt(4) Conexant/Intersil PrismGT SoftMAC USB IEEE 802.11b/g wireless network device
ural(4) Ralink Technology USB IEEE 802.11b/g wireless network device
urtw(4) Realtek RTL8187L/RTL8187B USB IEEE 802.11b/g wireless network device
urtwn(4) Realtek RTL8188CU/RTL8192CU USB IEEE 802.11b/g/n wireless network device
zyd(4) ZyDAS ZD1211/ZD1211B USB IEEE 802.11b/g wireless network device