There's heaps of F2P MMO's around now, even AAA quality ones like Lord of the Rings Online and Everquest II. I suggest checking them out! Also, if you are in to simulation and real time strategy, Age of Empires Online is a really nice game.
It all depends on how much space you consider "crazy amount". But most range from around 4GB-10GB, which is really not much at all in this day and age, what with really cheap HDD's and crazy Internets.
In any case, check out
Online MMO and MMORPG Gamelist, News, Reviews and Community at MMORPG.com as they do reviews for pretty much any MMO under the sun, including browser-based.