Spring is awesome, I have all the original Total Annihilation games (yes, even the Core Contigency and Battle Tactics expansions worth over $100 on amazon due to rarity) and Spring with XTA mod beats any RTS I have played yet.
I'm not downing Starcraft, I still have fun playing that game every now and then, but SC sells for $20 at Wal-Mart still. When a game has hit the age of 5 years plus then it's time to let go, but Blizzard are known to be greedy with their older games.
A good free game...Well, I'm a Linux guy and definitely love Battle for Wesnoth. DoomRL runs on Linux too and is awesome! Get over the graphics and the simplicity of the interface with keyboard only controls and DoomRL actually lets you play faster than graphics base games IMO, and it's still more fun and half of the games I played.
I could mention emulators and SNES Roms that are free to download, but there is so many games there for free it's crazy. All you have to do is buy a Logitech Precions Gamepad for $8 on Amazon or somewhere and you got endless entertainment of the 90's.