In Sluggy Freelance, when storming Hereti-Corp head-quarters, Oasis went from being unarmed to "sorta unarmed" after she killed two mooks with pencils.
See also the fight between Gwynn and Oasis, where the former uses her telekenetic powers to attack Oasis with... well, anything at hand, including the chunk of wood with which the assassin gets impaled, before being knocked off a cliff.
In The Adventures of Dr. McNinja, the the title character's ninja clan were formed from Irishmen who fought off pirates with frozen shamrocks, which they used as shuriken. At one point, the doctor recreates the trick... though as he does, he laments, "This is so retarded."
Also, Higgs is lethally powerful and accurate with a wrench.
Klaus himself is not above using non-combat units when he hasn't got any siege weapons ready. Either he taught Gil to think the same way, or their talent for compilation and adaptation of ready solutions makes it automatic.
Goblins. MinMax has "Weapon Proficiency: Furniture" (traded in his ability to wink).
As well as spectacular acrobatics and surprising blood pressure strength, Lycosa occasionally uses stuff she finds lying around to beat the crap out of her enemies. The twist is that she - and her opponents - are spiders. Therefore, a piece of glass becomes a huge sword, and a pill becomes a gigantic club.
Adrestia from Morphe makes good use of a broken floor tile.
When he was moved to his new maraca body way back in book one, Ennesby of Schlock Mercenary was given some very powerful localized gravitic manipulators to act as hands. He figured out a way to turn it into a maser weapon by focusing background microwave radiation, and he's been using it on and off since then.