When musicians aren't busy writing, recording and touring with their main band, often times you'll find them in side projects, mixing it up with new sounds and new collaborators. The 1990s were no different, as folks from the biggest bands to lesser known indie artists often found a new creative outlet outside their main gig. But what exactly makes it a side project, as opposed to just putting out a solo album, or recording with a fabled "super group?" We try to lay down some (admittedly) shaky criteria to figure out what makes a side project, revisiting those that worked, a few that didn't, some that left us scratching our heads and some that left us wanting more.