I've got a recommendation!
Title: By The Blade
Author: racethewind10
Rating: PG through NC-17
Summary: In the Country of Florida, the Miami-Dade Mercenary Company is the most powerful armed force. Led by Captain Horatio Caine, they protect their homeland... for a price. Calleigh Duquesne is Lieutenant and commander of the Horse Archers and Natalia Boa Vista is a noblewoman from another country escaping the abuse of her husband, Baron Nicholas Townsend. On a scouting mission into enemy territory, Calleigh comes across Natalia trying to escape and vows her protection. This is their story.
Warnings: Some adult language, descriptions of abuse, sexual situations.
Link: As some of the chapters pull in a NC-17 rating, you'll have to PM me for the link.
This might just be my favorite piece of fan fiction ever. It includes characters from all three CSIs, is set in an entirely different time period, and is just overall pretty freakin' epic. (Heck, it practically IS an epic with a whopping nineteen chapters!) I do a terrible job of describing how awesome the piece is, but trust me - it's a good one!