As I recall, certain of Zelazny’s Amberites spent quality time as mercenaries, as well as serving in standing armies. Moorcock’s Elric and Moonglum are often soldiers of fortune. Fletcher Pratt’s The Well of the Unicorn prominently features mercenary companies. The mercenary company is itself the focus of Glen Cook’s The Black Company. And, of course, more recently, companies of sellswords are significant players in George Martin’s Song of Ice and Fire. Ramsey Campbell’s Ryre is a mercenary, though he’s usually in between wars, fighting some supernatural horror or other.
The condottiero and the sellsword are useful narrative figures. No politics need necessarily drive the story. The mercenary isn’t fighting for a country or a cause. The conflict itself is the story, not the reasons for the conflict. There is often a presumed amorality attached to the sellsword, thus there is little need to establish right or wrong between the belligerents, and the author can get directly to the action. Time can be spent on individual qualms or internal disputes, rather than on geo-political or social issues. It’s a good setup for a writer of swords-and-sorcery. I’ve used it myself.
No doubt readers of this web log can think of many more examples. Are there some outstanding examples you can think of? There could well be one I haven’t read yet.