Actually, one of the most fun things about Temple of Elemental Evil is a strange bug (Easter Egg?) that allows you to turn basically any humanoid-ish monster into a permanent NPC. Basically, the Suggestion spell turns humanoid monsters into temporary NPCs (who only listen to you if they miss Will checks or something). This means that any goblin, troll, ogre, hill giant, gnoll, bugbear, or even giant tick or will o' wisp can be controlled by you; strangely, once they are under your control, they can gain levels and classes (you can choose their first one), which is loads of fun.
To make them permanent NPCs who always do what you tell them to do, you have to let them die (or do it yourself, but with only one hit, or they will revert back to enemy status), which will leave them in your party as permanent members. Then, go resurrect them (ask a priest or do it yourself - the former is often cheaper), and they are yours for good!
The bad thing is that NPCs in Temple of Elemental Evil always loot things randomly (items and money); you cannot take this stuff from them, either, and they will sell stuff whenever you go to a store.
Still, it is fun to have lizardman fighters, goblin druids, and so on; some items cannot be looted later, so if you want a lizardman with a lizardman sickle or a troll with a troll fork, you have to grab critters who already have them equipped. Some critters are not very good to have, though (giant ticks cannot equip anything, are weak, and take forever to move around); the will o' wisp would have been awesome, but unfortunately it disappears when it dies, so you cannot resurrect it and thus add it to your roster!
The final bad thing is that most townspeople will not have anything to do with you if you have ogres, trolls, etc. in your party, so if you want to do quests and such, do them first
