The main mechanic that I would like to find is where casters have a resource pool. Spells have a resource pool cost. Most importantly, this resource pool renews each round. So, is there a product for a magic/special ability system that does something like this?
@Nohwear: Yes. Kind of. Ethermagic by Interjection Games has a balanced, superbly crafted manabar-style casting. You regenerate etherpoints each round and can cast constantly, but you'll run out of EP if you don't take care. One note: To maintain balance, that does not work with regular spells (which would be ridiculously OP), but instead has its own massive array of customizable etherspells.
Ethermagic is contained in the excellent "Strange Magic" hardcover and also can be found as its stand-alone version in the "Ultimate Ethermagic"-pdf.