When Linksys released its firmware as open source in 2003 for the WRT54G-series router/wireless access point, numerous open source projects followed. Ten years later, OpenWrt and DD-WRT offer improved firmware that can supercharge old wireless gateways (Linksys, Netgear, Belkin) or turn a $50 consumer access point into an enterprise device.
These upgrades come with command-line interface, Web GUI, VPN, many wireless versions, and features such as mesh, WDS, OLSR, 802.11s, and BATMAN (Better Approach to Mobile Adhoc Networking). You can download bandwidth monitoring options, captive portal options, IDSes, QoS, Samba, Telnet, SSH, IPv6, and so on. OpenWrt has been the more active project, with more frequent updates and more add-ons, but both are quite excellent and have strong followings.
-- Joseph Roth