Starship Operators I enjoyed because of its attention to detail - particularly things like physics in movement and acceleration in space (I was a bit of a physics buff in school, and I still retain some interest) - it focuses on a war between an interstellar empire and a single renegade ship - all human, no aliens. It was disappointingly short though.
Kiddy Grade (very misleading name if you ask me) follows two agents of a government organisation - and it starts off with a case per episode type deal, but develops a very twisty plot later on, and you need to pay attention if you don't want to lose track. Some pretty characters, a fair bit of fanservice, and very high-quality animation.
Crest & Banner of the Stars covers a number of series - and its more of a space opera - it's got a very big universe and history to play off against. I watched it quite awhile ago, so I can't really remember specifics, but I enjoyed it quite a bit.
Planetes follows a group of space-workers in charge of clearing space debris from Earth's orbit - more of a drama type, so if you're looking for action this won't be your style.
Stellvia of the Universe is a little more easygoing again - not heavy on the action - and follows a group of students who are in the Space Academy in order to protect Earth from some form of interstellar catastrophe - after the planet being hit by one before.
I just realised how difficult it is to find sci-fi anime that doesn't have some form of mecha in it ... the Japanese seem to love the damn things.
Also ... I wanna go watch all of these again now. Damn you Ozerc0n
