take a gander at
http://spacesimcentral.com/Their forums have links to lots of amazing old, new, and in-development games of every kind -- from blockbuster to single-developer basement projects.
I've been meaning to pick up Starpoint Gemini (
http://www.starpointgemini.com/) which looks like good casual fun. Also, Captain Jameson might have promise (
http://www.farbs.org/SupportCaptainForever.html) and if you like more strategy 4x type, I've always meant to check out Light of Altair (
http://www.saintxi.com/)
I know its pretty lame to recommend three games that I havent even played, but I guess I spend more time reading about games now than I do actually playing them. I'll second some of the above recommendations. Transcendance is a great roguelike in space. Not the best game engine (sprite based ships with choppy rotation, and hard to aim), but great random replayable gameplay. Space Rangers 2 was fun, but I think it tried to wrap too much into one game. all the sub-games seemed a little thin. I know you said no X games, but maybe some of the mods would bring you around?
Ad Astra is a forgotten little game, but it was a good one too.
http://www.a-astra.com/index.htmI played Naev for a little while, too. It looks like development has come a long way, and I should pick it up again. Was a good 2d (with 3d ships...) space combat / trading game.
http://blog.naev.org/Probably no one remembers Star Raiders, but it was one of my favorite games back on the old 800.... Anyway, I see there is a remake now, which doesnt look half bad. I dont have steam, nor do I want them deleting my porn, so I havent picked it up yet.
http://www.atari.com/starraiders