HardwareSupport
Introduction
Pre-configured Systems
Hardware Components
Other Resources
http://www.fsf.org/resources/hw is a site that lists hardware that is fully compatible with linux. Good if you are looking to purchase or buy.
A large database of Linux-compatible USB devices and the drivers which they require can be found at The Linux-USB Device Overview Website
The Debian PCI device driver check page can check if most of your PCI hardware is supported under Linux. Run the lspci -n command in a terminal and paste the output into the box on the website. Then, press the 'Check' button to run the check.
Hardware4Linux is a collaborative site to report or find the status of support under Linux for all Linux distributions.
UbuntuHCL.org is a free and easy to use hardware compatibility list for Ubuntu Linux.