I got quite a decent deal on a one-way rental in Croatia from National Rent-a-Car's website. I didn't even know these guys did overseas business, but they appear to be all over the place. I had also checked Europcar (preposterously expensive) and Hertz (no one-way rentals available).

I've also gotten better rates in smaller places by just doing a web search for "X country" + "car hire" or "car rental". Via this method, I'll be renting from a local place in Inverness, Scotland next week for a far better price than the major chains could provide.