I compiled most the information below in a post on the OpenWrt forum.
TP-Link Archer C7. OpenWrt Wiki page. Must find v2 for 5 GHz support.
Netgear WNDR3700. Uses Atheros wireless (FOSS). Ships with a modified OpenWrt 7.09. Support in OpenWrt trunk is ongoing. It's an open question whether the router is flashable or not. Exploration efforts are documented. 2.4 GHz is supported on the WNDR3700 with OpenWrt 10.03, 5 GHz work is ongoing.
Ubiquity Networks SR71-A. mini-PCI dual-band 802.11abgn card. Uses Atheros AR9160 radio.