DD-WRT is the most popular third-party router firmware. People flash their routers with it to enable additional features and have a more intuitive interface. DD-WRT is important to the VPN community because every device connected to the router can benefit from the VPN. There used to be relatively few routers with DD-WRT support. Most decent routers now support the firmware, and manufacturers like Asus even advertise some routers as being DD-WRT capable. A basic Google search will usually let a user know if a particular router is capable of using the firmware. There is also a comprehensive wiki with a list of supported devices.
The amount of flash memory is the most important factor in determining if a router can use DD-WRT. Fortunately, it doesn’t take much, and the developers have made different sized versions to accommodate the widest variety of equipments.
2MB Flash – Not Supported
4MB Flash – Works for most basic installs
8MB Flash – Allows users to load additional software
Anytime you plan to flash a piece of hardware make sure you do your homework. There are few things shittier than plopping down $100+ on a fancy new router only to brick it shortly thereafter. DD-WRT.com is the best place to find every possible details about the firmware. The amount of documentation available makes the process not nearly as scary as it once was. Like anything else, you will never learn unless you try, and this is a case where the benefits are well worth it. That said, it might still be worth it to do a test run on the 5 year old router sitting in your closet first.