I compiled most the information below in a post on the OpenWrt forum.

Routers

Linksys 160NL. Single-band 802.11n only.

Netgear WNDR3300. Uses Broadcom wireless (binary module), somewhat supported by OpenWrt.

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 write my essay. Exploration efforts are documented.

D-Link DIR-825 rev B1. Uses Atheros 9220/9223 radios, and Atheros 7161 CPU. Supported by OpenWrt trunk (blog post with details by certified resume writers). 1000 Mbit switch supports jumbo frames. Dual external SMA antennas. 32 MiB RAM, X MB flash. D-Link DIR-825 B1 Quick Review

Linksys WRT400N. Atheros-based. 32 MiB RAM, 8 MB flash.

DIY kits

Boards

Ubiquity Networks RouterStation Pro. Netgate sells a nice board, enclosure, and power supply kit, as does Anaptyx Mesh.

Radios

Ubiquity Networks SR71-A. mini-PCI dual-band 802.11abgn card. Uses Atheros AR9160 radio.

SparkLAN has a good selection of mini-PCI and mini-PCIe radios, with Atheros and Realtek radios. Recommended from OpenWrt forum.