Spring (previously known as Total Annihilation: Spring) is an open-source RTS game/engine modelled in the likes of Total Annihilation. In fact, if you still have your disk laying around somewhere, you can use the original units from the game. If you're feeling adventurous, you can even add your own as Spring seemed to be made for customization. It actually incorporates some features that were used in Supreme Commander (like the rediculously zoomed out tactical map).
If you can get the game up and going easily (my first try was on linux, which apparently was harder to do for multiplayer), it can be quite fun for multiplayer matches.
Some gameplay vids:
http://youtube.com/watch?v=gIlEya0qT_E&mode=related&search=http://youtube.com/watch?v=-dN5zgKsJXQ&mode=related&search=http://youtube.com/watch?v=EzhEZOYICWg&mode=related&search=the website:
http://taspring.clan-sy.com/
