Hmm I think some of you have a mild misconseption of what a game engine is.
Halflife quake3 and such are not open ended game engines you cant take them and make a new game with them, what you can do is make a half-life or quake3 based game, ie a Modification or MOD for short (TC\'s or total conversions are possable but the basic gameplay and layout remain essential the same as the original product, TC\'s are more like removing all the visable content and redoing it and adding a few features (Sleep Walkers TC for UT and TeamFortress TC for Halflife)).
The only way you can use those engines to make a brandnew game is to get your hands on teh source code and alter that, useing there exsisting network and graphical engine code as a basic framework, although the programming level and amount of re-programming required for such a feat is far in excess to what most normal programmers have (Im meaning the beginner and intermediate programmers).
Engines like the Crystal space engine, Nel, Nebula, WorldForge and such are the class(?) librarys and basic framework open source engines designed with the scafholding of a engine in place such as the graphics engine, sound engine and such and you have to code the rest of the content and/or tweak them to your liking. Kinda like buying a half a car engine and a few parts of the chassey then making all the rest your self, gearbox, seats doors, fuel tank (The Half-Life and Quake3 engines are more like a car just missing the outside metal shell).
Im not saying the half-life engine cant be used to make a game like a MMORPG you\'d just need to buy the licience to the source code and that would cost you 3000$+ , youd then have to drasticly alter the networking code to make it run like an MMORPG. Redo all the game content and such. in many situations the engine are programmed to much into there base game type, so it would be just as fast and much less work to get a free all incompassing basic engine like CS, Nel, nebula or programme one from the ground up your self.
Basicly you cant make an RPG from half-life, you could how ever make a medieval FPS RPG-like Half-Life MOD or TC.
Heres some links to game engine resources.
List of some engines, not many usefull ones in there...
http://www.ambrosine.com/resource.htmlWorldforge..
http://www.worldforge.org/Nel... (i particulary like the look of this one)
http://www.nevrax.org/Some more game engine lists
http://www.gdse.com/servlet/gdse.links?s=1Nebula Engines homepage..
http://www.radonlabs.de/