A friend of mine uses his Vaporeon in a very effective way, but when you look at its moves, you can tell its has a solid weak point; other Water Absorbers. (not to mention Special-based Electric types) But when he's used it, he's almost always managed to pull through for the victory. (an example currently on my profile for reference)
Other than that, Glaceon can also be a very useful Pokemon; particularly against those pesky Dragon types. I've trained a few, but I've never actually used one in battle, yet... but soon!
Bibarel never gained any kind of usefulness outside of being a slave for HM moves, but it was useful, nonetheless.
Again with examples from my friend, Jirachi is quite a supporter; learns both screens and with Platinum, can learn Iron Head.
Smeargle is another good Pokemon; primarily for breeding Egg Moves, and such.