GURPS is the Generic Universal Role Playing System from Steve Jackson Games. GURPS 4E is a skill-based point-build RPG focusing on flexibility. You can easily play anything from a stone age caveman, an ogre mage, a US navy soldier, or even a Zenbuddhist psychokinetic killer robot from future (who likes cats). Crossovers are easily to handle with those rules. This system uses six-sided dice and hexagonal map-grids.