Kerbal Space Program is awesome, and has just got much more awesome with a huge recent update that amongst other things has introduced docking between craft.
This massively increases the possibilities for amusement and flaming Kerbal death, but also makes the whole thing seem a little daunting to some at the start. Rock, Paper, Splashdown has several Kerbalnauts from the RPS Steam chat - MFToast, Jon "Moose" Tetrino, myself and others - who're happy to offer advice on getting to grips with KSP.
Docking ships in orbit lets you construct craft you wouldn't be able to lift off the ground from the launchpad. This means refuelling or constructing interplanetary ships in orbit - making landing bases and planning more complicated missions to other bodies much more possible.
It also means awesome space stations.
KSP has no simultaneous multiplayer option, but Jon and Toast are planning on a shared, online save file for RPS space stations, Mun bases, and whatever else anybody wants to do.
Get the game and join the group! Launching enormous rockets through space is cooler than anything else you'll do in a computer game today.
(NB all screenshots murderised by the Steam screenshots system, 'scuse the quality.)