Warcraft III

An awesome fantasy RTS. A great campaign with a solid story line. Cunning bots to practice with in custom games. Competitive multiplayer matches on Battle Net. In depth world editor for custom stories and matches. You can't get much better than Warcraft 3.

You can also get the Frozen Throne expansion for a new story and other cool features.

You can buy this game on Blizzard's website: Warcraft III

On the website, it says it requires 10.6.8 to run. Do some research so you can find the right version. If that's not your style, there are torrents out there...