Ragnarok Online

Generally an all-right MMO, with a fun PvP-system that includes both standard PvP (Player vs Player); as well as WoE (War of Emperium)

What's WoE, you ask? Why, it's a semi-daily event that has various guilds of players competing with one-another for the possession of each others' castles. Basically: you scale your way through the enemy's castle, kill the enemy guild's guardians and defending players, and lay siege to the Emperium - the very heart and lifeforce of a castle - attempting to break it so you may claim the castle as your own.

Why bother? Because castles give you numerous amounts of benefits, such as: a guild storage, an economy system that'll reward you with rare items and gears, and even your own personal dungeon for when you're having difficulty hunting for those pesky, rare monsters. There's also bragging rights, which are awesome for when you single-handedly take down an entire guild of 72 players by yourself (true story).

An example of the chaos that can ensue during a WoE with many participating guilds (ignore the shitty music and ventrillo chat). Needless to say: WoE can be fun as hell.

While normally the game is p2p (pay to play), there're private servers that'll allow you to play the game for free. A private server is essentially a server using eAthena software, an emulation of Ragnarok Online and such, which constantly updates with new scripts related to RO's most recent updates.

Here's a site that lists some private servers, which acts as a database for the game as well:
http://ratemyserver.net/

So, does anybody else play any private servers? I'm currently not playing any, due to my previous two servers suddenly deciding to stop their updates.