Eve, Heimer, Mundo, Karma, Miss Fortune, ect. But I believe they all have their reasons for not being played due to balance issues.
Although there were 3 champions that stuck out for me and I cannot figure out why people don't play them more often.

Trundle

I don't usually jungle, but when I do, I play Trundle. If you are new to the jungle he is a very good champ to learn your way around it with. His quick and easy jungle route makes him able to start ganking very early. Getting his pillar at lvl 2 makes for strong ganks for just about anyone. At least making them waste summoner spells early on. During Mid-game he acts as a strong ganker, once the pillar is max out, the slow from the pillar is extremely slow and also acts as a wall, if placed right it will ensure many kills for you. Also his "W" Contaminate is a speed enhancer that increases both movement and attack speed that also reduces all CC affects while in the circle, so he can get in and out with the quickness. Building him tanky dps with tri-force makes him nearly unkillable late game and still puts out a very high ammount of damage.

Galio

I really don't know why I don't see more of him solo top or even mid. After the last buff to him he is nearly unkillable to any of the AP carries. His passive is in my opinion the best tank passive in the game (Runic Skin - Galio converts 50% of his total magic resistance into ability power.) That means he can build mostly Magic Resistance items but still will be able to deal massive damage. Just that alone is why people should play him more, although its not his only perk. He has one of the best ultimates for team fights in the game...Taunts all targets in the area of effect and at the end of its duration explodes doing damage to all targets. Just his ultimate alone can change team fights around, but combined with other aoe ultimates, such as Fiddlesticks, Nunu, Amumu, Annie, or even Karthus is pretty much GG if all used in Harmony. Note: Galio is a great last pick if the opposing team is AP heavy!

Nunu

There is nothing worse than taking a max Lvl Ice Block to the face. AP jungle Nunu is one of the best gankers in the game but often over looked as a viable ap jungle, his Consume makes jungling and counter jungleing a breeze. One of my good friends has been playing AP jungle Nunu and let me tell you, he has carried multiple games with him. Once you start getting some ap items Nunu's Ice Block starts hitting like a truck, and slows them for easy ganks. His blood boil allows him to run into the lane and Ice Block before the target can even react and before they know it they are in the middle of a blizzard of doom! Again, when combined with Galios ultimate it gets a bit ridiculous. He is said to fall off late-game, but honestly from the games I've seen lately, that is not the case at all.

I give you all a challenge to go try these champs out, let us know how it goes and maybe post some screenies of the final score! Good Luck!