dd-wrt is open-source linux-based firmware that can replace the stock firmware on many wireless routers and it is awesome.

I use dd-wrt with my dirt cheap ($22) Rosewill RNX-N300RT router and it takes a low-end router and makes it a powerful tool for managing my network.  Some of the key things I like about it include:

• Separate WiFi guest network that is fully isolated from my home wired and wireless networks
• Fine control over hours of usage by Wired and Wireless clients (helps limit kids’ access to reasonable hours)
• Superb web-interface for diagnostics and administration
• Excellent firewall

dd-wrt maintains a database of routers they support; if you are technically inclined and your router supports it, I highly recommend trying dd-wrt; note that you almost certainly void your router warranty by replacing its firmware.