All enemies drop their weapons after they're killed as well, but I've had some trouble finding enough ammunition, so you'll be forced to change your weapons pretty often. (softpedia.com)
Some enemies also drop weapons, which can be used against them and their various cohorts. (superphillipcentral.com)
Your fire power will start of quite weak but destroying certain enemies will drop weapon power ups, a drone and even a force field. (insert-disk.com)