Spring started out as a mod for Total Annihilation. While there are other games available on the engine, most of its user seem to play Balance Annihilation, which is heavily based on the original TA (it uses the same models, but the controls are refined and various units have been altered slightly to provide more balanced gameplay). It's not only a clone, but actually an improvement — the AI being the biggest leap, really.
I won't touch on the legality of the game.
There's Streets of Rage Remake, which is essentially a remake/fusion of all three games with tons of bonus content and extensively redrawn graphics. Of course, it's no longer available at the developer's site due to being C and D'd, but it's still out there on the Net for people to download.
Black Mesa is a complete remake of Half Life 1, and unless they've changed something, it's free on Steam. The levels in Xen aren't done yet though (probably in an admirable effort to make them less of a drag as in the original) so it's only the game from the beginning to the 14th chapter, just as you jump into the portal to Xen. I'm curious to see how they'll extend the last 4 chapters of the game into something lengthier.
Does Frets On Fire for the Guitar Hero / Rock Band-games (the guitar part, that is) count? It's basically an Open Source version of the Guitar Hero-mechanic that supports using the original guitar controllers.
Adventures Of Dino Riki: The screen is constantly scrolling forward, so even if you're standing still, you're always moving forward. So if you're on a small or otherwise temporary platform, you'll need to keep tapping backward to avoid falling to your death.
The system doesn't know you right now, so no post button for you. You need to Get Known to get one of those.