Well, it
is free, but you have to have an Nvidia Gforce based card, and you have to be running XP/2000. Sorry, I didn't want to put all of that in the topic
I just noticed, upon downloading the latest detonator drivers, that there is a monitor rotation program built into it. It's called NVrotate. I haven't tried it with mame, but I think it would probably rotate your already rotated games. Might take some tweaking to get it to work, but I am sure somebody out there will give it a go. I have tried it to just rotate my desktop and it works fine. It says that it is made for those rotating LCD monitors, but I used it on my 19" CRT. No problem.
http://www.nvidia.com/view.asp?IO=feature_nvrotatestonefry
P.S. Haven't been here in a long time. The new forum looks pretty dang sweet.