Has anyone suggested abandonware yet?

If you like RTS, old, cntrl-click interface for multiple-unit selection, you should consider the original Warcraft. Abandonia and go... You'll need DOSbox, but that's really easy (and free) to get a hold of.
If you like flight sims that are still shockingly realistic, check out Parsoft's old A-10 series. The only version they made for Windows was A-10 Cuba!; if you've got a Mac you
might be able to find a way to run Cuba! and Attack! using an OS9 emulator. If you can do that, you get all the fun options I never had as a kid (I never had A-10...

) like the mission editor, etc. The Windows version isn't as good.
If you find that abandonware is uncomfortable, I throw one last suggestion to the breeze: YS Flight Sim. Yes, it does suck, but once modded to use better, "more realistic" aircraft, it actually is quite fun. It achieves this feat by being so shockingly simple and almost arcade-like that you can merely focus on flying. Combat missions are also fun. This might be a good one for multiplayer...
To clarify, YS Flight is freeware. If you need to modify something, also note that most things are alterable via text editor. I've never tried making a new aircraft, but I believe there's tutorials out there. The source code is NOT open.
-Thaeris