Here are some of the most common found in a lifters bag.

A good leather belt - maximuscle do a really nice one for £30


Lifting straps for heavy work(back etc)


Knee wraps for heavy leg days


Other stuff includes smelling salts and a full on power suit for the big powerlifting boys. I don't bother with salts as they make me want to heave but a few guys at our gym swear by them. I think they even take carry them round with them and sniff them through the day

I try and only use the lifting straps and knee wraps on really heavy weights. If you use these things too much they will restrict a certain amount of development, e.g. if you use the straps on every pulling exercise you won't get the forearm benefits. On the flip of that, you are only as strong as your weakest link and so if your forearms are weak, it could restrict back development. Only you will know if you are working your back hard enough without them.