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.
D-Link DIR-825 rev B1. Uses Atheros 9220/9223 radios, and Atheros 7161 CPU. Supported by OpenWrt trunk (blog post with details). 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.
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.