So recently I put the game Dungeon Fighter on my Asus Netbook. Dungeon Fighter is an interesting game – a side-scrolling beat-em-up game and an MMO at the same time. You play one of several unique classes (that at times vary from fantasy archetypes or expand them), fight monsters in modular dungeons, and have colorful sprite-based fun. It's easy, simple, surprisingly deep, and the Priest class whacks enemies to death with giant crosses, scythes, and rosaries, so how could I resist.
The game is of course free-to-play, but you can blow cash on getting extra equipment, respecs to re-build your character, and, of course, character clothing so you don't look like everyone else. Very standard model.
So as I played this game, I debated if I wanted to get some credits in the game for extras. It suddenly struck me that the freemium, free-to-play, and other free-but models differ from the usual monthly-charge MMO games in another way besides the obvious.
They allow you to timeshift your expenses.