This. Age of Empires II was my favorite strategy game for years - now it's StarCraft II. You could play that game FOREVER. The campaign is actually pretty good too.
Age of Mythology is in my top 5 for sure! LOVED that game. Way more fun than Age of Empires for me, even though it played exactly the same. Unfortunately, it was never ported to the Mac properly if I remember correctly.
I second Starcraft II for a great RTS. If you want something a little more slow paced, but still in the vein of the Age series, you could try Star Wars: Empire at War. Playing Boba Fett in space combat NEVER gets old!
I am also a big SCII fan. A few points to note, however:
There is an expansion coming out soon (currently in beta), after which the price of the original game ("Wings of Liberty") should drop.
The multiplayer RTS element is demanding. It is not in any way like a turn based strategy game. Serious time commitment is required if you want to move up the multiplayer rankings.
If you get into it there is planned third installment to look forward to!
If you are less interested in working on your APM and want a relaxed MMO, WoW is always a safe bet. It increasingly caters to casual players.
EVE Online is probably also worth a mention if you want an MMO with a strong community.