It happened: I've found a free to play game that I not only enjoy, but am constantly tempted to throw money at. This game is Age of Empires Online and I'm totally into it right now.
I played the original Age of Empires a ton as a kid. I wasn't very good at it, to be sure, but I liked the historical theme and the music. And I really liked hunting down and harvesting meat from elephants. Seriously, by the end of any game the elephants would be long gone. I think I even almost convinced my parents it was educational.
Fast forward to 2011, when Age of Empires Online first came out. It was free to try with two civilizations with many locked features and $20 for a license of the game, which included a premium civilization with those before-mentioned features unlocked. I heard it was pretty fun from a podcast but I didn't really get into it.
The gameplay is a mix between an RTS and an MMO. You have a capital city that is your central quest hub including inventory buildings, stores, crafting, and even dailies. After you accept a quest, you carry it out by going into an RTS game and playing as you would a normal Age of Empires game. But instead of just going from one game map to the next like the older games, you go back to your capital city for rewards and exp and such, and then choose what you want yo do next. Lots of these missions can also be done co-op with your friends! You also end up making alliances with other cities (you can travel to these for more quests/dailies) and there are neutral areas like Sparta (this is where you queue up for a game against another player) and another one who's name I can't remember which is where you can do skirmishes against the computer (also possible with a friend).
Now, it's truly Free to Play with a premium currency (Empire Points) that you can buy (charged through Steam) or earn very slowly in game to unlock things. More importantly, it's also on Steam now, which means awesome deals from time to time. That's what got me back into it. There is currently a 1st anniversary deal that includes a premium race (the Celts), the PvE skirmish "booster" (booster is their word for non-civilization specific content), expanded inventory space, and some other cosmetic stuff for only $10. Anyway, it was a good deal, and playing as a non-premium race isn't actually that fun because you can't equip any of the particularly good items. (Note, you can also get a Greek premium race on Amazon for like $4 right now.)
All told, it's a really fun new way to play an RTS and I'm having a great time with it. I really like the "level up" approach and the more social feeling of the game. There's also a new, cartoony, art style that is kind of fun (but not amazing...except for some of the animals, they have awesome animations) but will probably help it to not look so dated in coming years. Let me know if you play and we can do some co-op.
P.S. If they made a Starcraft game like this, I'm pretty sure it would be the last and only game I ever played.