Don't forget the HL1 Source Engine was based on tada a modified Quake Engine
and Valve got all their knowledge by makeing Modifications for it so yeah ID had the bigest impact by starting the Modification Community, and that brought alot of smart guys to the Table and basicly helped them to get their own ideas out to the Public.
ID still gives that knowledge away freelie by releasing their sources to the public regular after the EOL of it.
Crytek is a very intresting Company, because of it's multicultural touch jesus the're working so many People from so many Nations just look @ their Website and see all the Guys and Girls.
And yeah you see Yerli also said that he worked on a Modification based on Unreal (does somebody know wich mod he actually worked on ?).
Engine wise i would say the Tech Engine and Unreal are the versatilest and most licensed Engines existing, but the're alot of new Engines showing up on the floor and Cryengine2 is one of the most compeling ones but it's still very young and first has to Prove itself in the Market especialy as it's mainly done for Outdor stuff and all Games that have been released so far with it showed that special use case like FarCry, Crysis and next FarCry 2 so let's wait and see what exciting ideas maybe their Mod Community come up with.
Another very interesting Engine that wasn't named yet is Gamebryo, actually it's used in many titles today but many don't know it hehe, another Engine that really shouldn't get out of the Focus even if it isn't talked alot of anymore (but it's the most used on a daily base Worldwide via 3Dmark) is definately the MaxFX-Engine wich new Development status we should see with Alan Wake soon.
Every of those has their very own Rendering Look and Feel as one for example mentioned here Tech Engines Clay Look and Feel another interesting Look and Feel i would say is the Plastic one from Monoliths new Lithtech Engine used in F.E.A.R. Vales evolved Source Engine has a very own Look and Feel too that is hard to define for me it feels like it's more in the comic section the Characters doesn't really look Real even if they Act like being Humans, strange to define it for me in words hehe. CryEngine2 tries to be the most Photorealisticst Engine but the Characters still look not quiet there yet