Ragnarok Online
Ratemyserver has a good list of the free servers. As I've mentioned in the RO thread, please read the server descriptions and reviews before joining a server. Here's a small list of things to look for:
Rates (experience and item drop rate)
Population (want to play on a 50+ or 300+ server?)
Special NPCs (easy job changer, warper, healer, etc..)
Up to date (is the server owner actively improving the server?)
Custom items (sometimes they can really ruin the balance)