DD-WRT firmware: One of the biggest advantages of DD-WRT firmware router is the huge community behind it which makes it easier to get help. Open source DD-WRT firmware is free for individuals but if you want to use it for commercial purposes need to pay a license. DD-WRT has a multilingual interface, bandwidth management, it supports over 200 devices, all WLAN standards and it has VPN integration.
OpenWRT firmware: OpenWRT is a Linux distribution normally used at embedded device, OpenWRT offers many features not previously found in consumer level routers. There is support for DHCP, and wireless encryption in between many others. Open WRT allows for package management allowing users to install new software according to their needs.
Tomato firmware: This open source Linux core firmware distribution is used mostly for Broadcom chipset based wireless routers, it features an easy to use Ajax based GUI interface, bandwidth usage monitor, advanced access restrictions, wireless features, P2P connections bandwidth control, running of custom scripts, scheduling of reboots and much more.
WARNING: Flashing hardware firmware is dangerous and installing third-party firmware is normally unsupported by router manufacturers. Use it at your own risk!