Options for free software in Linux are:
is an excellent but unwieldy piece of software for use with RTLSDR. It's the only software for now that will receive analogue (FM, AM, sideband) and digital modes such as AP25, FSK and ADS-B. Be warned it's not very easy to drive but if you are canny you can use it to build your own software defined radio. Follow the build guide for your Linux system as the latest code is always the best to use.
apparently does too.
no longer works in Linux! :(
Have a look here for a good summary of Linux utilities.