For space combat sims, the Freespace games are pretty deep. Lots of controls to learn, lots of ways to approach most missions (Freespace 1 was better about this; in FS2, your character was shuffled between various squadrons as the game went on, each of which had a specific role they filled), and lots of toys to blow shit up with.
Wing Commander: Privateer would also be a good choice if you want a combination of a space combat sim and a "life on the fringes of space" sim. You can do missions of virtually any kind, in a wide-open setting. There IS a storyline you can follow, but it's not mandatory. (Be aware however that this is currently one of the games suffering Missing Expansion Syndryome™, although it's not that hard to add it in if you have the files.)
EDIT: Saw someone recommended Recettear: An Item Shop's Tale. I'll second that as well. You play the NPC item salesperson in a JRPG, wheeling and dealing to make money to pay off the mortgage your deadbeat dad left on you. I'd say pick up the demo on Steam, and if you decide you want it, I can give you a 50% off coupon I got during the Winter Giveaway.)
Post edited January 19, 2012 by boct1584