Irenaeus.: Oolite (
http://www.oolite.org/) is a free multi-platform port of Elite with modern graphics. You can add extra ships and missions via their extension packs (OXPs). I play it occasionally rather than fire up my BBC :)
I tried that, a very nice remake of Elite (sorry folks can't be the original)
Recreates the game wonderfully, and the OXPs are wonderful, beats most commercial Space sims dead in space. I have yet to see a game that handles mods as wonderfully as oolite (it's easier to use then the Bethesda games mods).
Vega Strike is another great one with lots of variety, the prebuilt campaign is pretty amazing for a freeware game and beats out most commercial titles. It also has some mods here and there, but it's not nearly as impressive as oolite in most regards.
I just got Evochron Mercenary and i will tell you guys what I think about this one.
Of all the Space Sims out there this one seems to adhere to the gameplay of Elite the most.