Linux supports most hardware now.  The only two major catagories of things it DOESN'T support today are cheap WinModems and Parallel Port Scanners.  If you have some really new or really tricky hardware and want to check if Linux supports it, you can always find the lastest hardware compatibility list at http://users.bart.nl/~patrickr/hardware-howto/Harrdware-howto/Hardware-HOWTO.html  Keep in mind that this list is conservative, and things are usually only added as compatible after testing.  I have at least two pieces of hardware that this list says is unsupported and they work fine with current versions of Linux on my machine.  (a Zytel pcmcia modem in my notebook, and a Lucient WinModem on in my desktop)