Free FIRMWARES for Home Routers such as TP-LINK or D-LINK and CPEs such as UBNT, Mikrotik etc.

Warning: Installing a custom firmware can cancel the warranty of Router provided by the manufacturing company.

Note:  Pretty much any Linux-based firmware requires at least 4MB flash memory.

=> Below are the features that the alternative firmwares may Offer:

1. QoS – Quality of Service i.e Traffic Prioritization. 
2. Wireless Bridging and Frame-bursting.
3. Ability to turn into A Hotspot.
4. Support for resource monitoring via SNMP.

5. VPN Tunneling Support.
6. Web Filtering and Ad-blocking Functionality.
7. Syslog and DNS Caching.
8. Support for creation of multiple SSIDs.
9. VLAN Support.
10. Repeater and Extender Mode. 
11. L7 Packet Classifier
12. Options to max-out everything, for eg TX and RX Power.
13. Port-knocking (with knockd/knock) and Intrusion Prevention System.

Here are the few Firmwares which are just the BEST.

1. DD-WRT: DD-WRT is by and far the most well known, distributed and tested of the alternative firmwares. The only downside is- not all of the features are available at free of cost.

Here are some notable features:

• Wireless Signal Booster. Guest SSID and Hotspot.
• QoS to limit bandwidth for individual users. Ability to create MAC based traffic Priorities.
• Detailed information about your connection speed and usage.
• Powerful filtering systems (MAC and IP filtering).
• IPv6 Support.
• Wake-on-lan.
• OpenVPN support.
• Support for custom MRTG and L7 Filters.

2. OpenWRT: OpenWrt is described as a free Linux distribution for embedded devices. Instead of trying to create a single, static firmware, OpenWrt provides a fully writable filesystem with package management.

OpenWrt is its fully writable file system, which allows for installation of updates without rebuilding and flashing a complete firmware image.

• Static DHCP leases. Wi-Fi Protected Access.
• Extensive firewall configuration.
• QoS for applications such as VoIP, online gaming, and streaming media.
• Configuration of the device as a wireless repeater, wireless access point, wireless bridge, or even a combination of the above.
• Mesh Networking.
• Command line access via SSH or telnet.
• On devices with USB ports, it supports printer sharing, Windows-compatible file sharing (via SAMBA), USB audio, and practically any other device that can be connected.
• An extensive AJAX-enabled web interface.
• Multi WAN i.e Link Aggregation - Load balancing traffic from more than ONE Internet Service Provider.

3. Gargyole: It's a completely free OpenWRT based firmware created for novice users. It is not meant for power users, so don't expect extreme flexibility as you get with DD-WRT or OpenWRT.

The primary goal of this project was to provide a polished user interface with features that other stock firmwares lack.

Features such as -

1. Moniroting Per User Bandwidth Usage.
2. Traffic Throttling.
3. Creating Wireless Bridge
4. URL filtering.

4. Tomato Firmware: Tomato Firmware is a free Linux-based firmware distribution for mostly Broadcom chip-set based wireless routers. 

Tomato features an easy to use GUI, a bandwidth usage monitor, advanced QOS and access restrictions, enables new wireless features such as WDS and wireless client modes, raises the limits on maximum connections for P2P, Wireless LAN Radio power of adjustment, antenna selection, and 14 wireless channels, allows you to run your custom scripts or telnet/ssh in, and do all sorts of things like re-program the SES/AOSS button, add wireless site survey to see your Wi-Fi neighbors, and more.