Another vote for the Akai LPK25 -- it's superportable, so it's easy either to keep right by your computer, or even to throw into a laptop bag and have along with you.
Personally I like Sonar (the current generation Cakewalk) on Windows as a software solution. I can't speak to Linux solutions.
I also keep some composition apps on my Android phone. If you really want to capture musical ideas wherever you are, a phone is perfect. I like Caustic and ULoops for Android, and I've also tried NanoStudio on an iPod Touch and found it quite nice.
Personally I like Sonar (the current generation Cakewalk) on Windows as a software solution. I can't speak to Linux solutions.
I also keep some composition apps on my Android phone. If you really want to capture musical ideas wherever you are, a phone is perfect. I like Caustic and ULoops for Android, and I've also tried NanoStudio on an iPod Touch and found it quite nice.