Technically a light novel, but it's manga-like enough.
Slayers
"Lina Inverse doesn't like to brag but she's a genius sorceress, invincible warrior, and a legend in her own lunchtime. Lina's also in need of cold cash. When she comes across some stolen loot, she figures the thieves won't mind sharing. But it turns out that Lina also snags a secret hidden in the booty... And now everyone's out to see if she's got the goods!"
Stolen from the back of the first book. Basically, Lina is an awesome sorceress, who just so happens to attract a ton of trouble. She usually ends up having to defeat vicious demons and dark gods trying to destroy the world, of course.
She's joined by her loyal bodyguard Gourry, the master swordsman without brains, and occasionally the mysterious chimera Zelgadiss and the love-freak princess Amelia.
It's written excellently, with very nice art pages. If you haven't even heard of this series, you're seriously missing out.