Remapping keys on keyboard in Linux

I am currently using xmodmap for remapping keys like Caps Lock on my keyboard.

However, my keyboard has a bunch of keys on the top that are rarely used (Multimedia keyboard with buttons like Internet, Music, Next song, Mute etc)

I want to remap these keys to specific commands (Not keys).

For example: Pressing the Internet button should fire up my Firefox (/usr/bin/firefox)(which happens only in Windows for me)
Also I need to map a specific button to xkill.

Is there a way to achieve this?
Using: Fedora 14 on GNOME and KDE (both)