Thinking of something like the MMORPG games use for building characters ... is there anything like that available outside of a MMORPG game?
HeroMachine is a quick-and-dirty full-pose character creator that I've successfully used to represent an entire party of fantasy characters and a variety of humanoid monsters. The fixed base pose makes a HeroMachine character instantly recognisable as such, but the variety of features and accessories makes it surprisingly flexible.
Actually, you might want to consider using some of those computer game tools that you mention. Ever since Eve Online was released (possibly even earlier), 3D character creation in computer games has become incredibly advanced. Check out the most popular computer RPGs of recent years and you will probably find several tools that allow you to create highly customizable portraits.
If you don't want to subscribe to an MMORPG, keep in mind that several single-player games offer these tools as well. Dragon Age is suitable for fantasy portraits while Mass Effect should work well for science fiction and modern-day settings. Note that both of those games have sequels, so I assume the later games have the most features in that department.
Taking a screenshot of a computer game isn't always as straightforward as pressing
Print Screen. For that purpose, several dedicated applications are available, such as Fraps.
Finally, don't forget that you can also take screenshots of your character in-game, giving him a suitable backdrop and some equipment to boot.
Campaign Cartographer has a supplement for doing just that: Character Artist Pro
I use CC2 and CC3. Love them. Wish there was a real Mac version... but it runs under WINE on Mac, Unix, Posix, and Linux, and natively on XP or later.
But I've not personally used CA Pro... but one of my players did, and it produced great output.
In-game faces for computer games are driven by software like FaceGen It falls quite short of a full fantasy character generator in its own right, but it's a lot of fun for other reasons. The free version is quite usable for exploring face shapes, or seeing what you might look like as a different race or sex. I believe there is something similar built into Poser, which is used by graphic artists to generate fantasy artwork, but which takes a lot of effort to learn and generate usable results from. Similar to Poser, but free software that you buy pre-built 3D content for, is DAZ Studio
Content warning: I've linked to inoffensive images that relate to OP, but there is much nudity in Poser and DAZ artist communities.
Besides what @Jakob suggested in a comment on the question (why not an answer? :)), you might want to check out the character creator part of The Sims 3 as well. I guess you'd have to take screenshots here too, but it allows for not only face construction-customization but for rather fine tuned "body definition" as well. (Then maybe photoshop the result a bit to fit your game world.)
If the SIMs isn't to your liking, you could try Second Life. additional benefit would be you could mold the surrounding environment. I suppose it depends on your point of view if using SL to make your character falls under available outside of a MMORPG game (I never considered SL an MMORPG), though if it does I supose that nixes SIMs.
You could try Ultimate Flash Face it's a free, online tool for doing "composite sketch" style portraits.
Obviously, this applies to human characters, but it is more "sketchy" than "cartoonish."
Adding to the list of suggestions to just use a game's character construction tool, I actually have used in-game screenshots from Morrowind and Oblivion for character portraits in the past and (depending on genre) Skyrim and Fallout 3 would also work quite well.
The main advantage of these games over an MMO and some of the other suggestions is that the Elder Scrolls and Fallout 3-series games have large, active modding communities with a ton of graphical mods available, so you're not limited to just using character types, equipment, or locations that are included in the game by the publisher.