EQ2 i think has a mentor like system. You can bring yourself down to the same level as others and take part in their questing areas, dungeons, grinding etc.
it's been a while since i have played it though, but it was a great way to run through an old dungeon that you normally couldn't get loot from due to being a lot higher in lvl. with so much content in EQ2 there was always quests and stuff that you ended up passing by and could anways go back and do with lowbies in this way.
Recently i have played a lot of Runes of Magic. Although they don't have a mentor system, they do have a dual class system.
Although this isn't the same thing, i was thinking you could save your secondary class for leveling up with your friends when they were online and play your primary when they weren't. each of the two classes you pick has their own xp and talen point pools so your secondary class will not gain in levels unless you set it as your primary.
there's more to it than that and it's a very cool system once you work it out.
oh, it's also free to play
Warhammer Online has a mentor like system in the game in the form of items that you can use to raise your friends up to your level.
It's not permanent and the items are also part of a "recruit a friend" deal or trial program, so not really something the average solo player will have access to.
those are the ones off the top of my head.
-roob