Most turn-based RPGs will have some form of "uber-mega-attack" like summons. Djinn in Golden Sun for example or those things from the Tales series (Tales of [Something]). It's usually in-lore limited to a set group of individuals to explain why everybody doesn't summon hell on everybody else, even if in-game all your characters just happen to be from that group of individuals...
It's also often implemented in more "freeroam" western RPGs. The Elder Scrolls series (Morrowind and Oblivion especially) lets you summon NPCs to fight with you if you have the spells and the skill (Conjuration), as do a some DnD based games like Neverwinter Nights (if you play as the appropriate classes).
Summonable allies (as in The Elder Scrolls and DnD) tends to be more of a western thing, whilst "uber-mega-attack" summons tends to be more common in Eastern games (like Final fantasy and Golden Sun) though obviously that's not a hard-and-fast rule.
