GURPS Ritual Path Magic

GURPS Thaumatology: Ritual Path Magic details an optional magic system for use with the GURPS tabletop roleplaying game. It is one of the most popular magic systems used with the game, and a stripped-down version is included in GURPS Monster Hunters.

It is an improvisational system, allowing spellcasters to make up almost any spell effect on the fly, but it is relatively complex in play. It can be contrasted with the Sorcery system, which requires players to select the spells a character has access to during character creation, but is much simpler during actual play.

Links:
Official Site
RPG.net Game Index