This Atheros-based (ath9k_htc) adapter works well as a BeagleBone AP:
http://www.sparklan.com/product.php?func=view&prod_id=197
You'll have to solder on your own USB connector or cable, though. Also,
it can draw up to 650 mA, so you'll need to power it from some external
power supply such as a powered hub.
As an alternative, this product supposedly uses the same Atheros AR9271
chipset, but I haven't tried it myself:
http://www.newegg.com/Product/Product.aspx?Item=N82E16833704046