I too want to know if this exists? Some game would benefit a lot from allowing you to lock the frame rate at 30fps. Such as GTAIV.
This: http://rapidshare.com/files/160642187/FPS_Limiter_0.2.rar should work with gta4 (dx9) I'd also like to see a dx11 limiter soon.... edit. ToMMTi seems to have a couple of nice tools: http://www.tommti-systems.de/start.html
There is such a thing as a FPS limiter, but it's built into any game/application that can possibly benefit from it.
Theres a workaround using Fraps and its "Flashback" cache feature. Just set the FPS you like in "Fraps Video Capture Settings" and set "Lock Framerate". I use my Ramdisk as storage for the cache file but you need at least 2GB of free space so you may use your HDD instead. When in game press and hold the record video hotkey (F9) until the FPS counter becomes pink. The FPS is limited and locked without Fraps recording a video forever but only the last 30 seconds when you hit the hotkey again.
You can also in some games use RadeonPro to have 60 or 30fps locked. Just use the force vsync option enabled. Enable "force tripple buffering", "force aggressive api detection level" and in the dropdown menu you choose "Force On- double vsync". Forks fine with some games ive tested, Dead Rising 2 worked fine with this written.
I was just about to fix that, I have both nvidia and ati systems so I forgot I was in the nvidia thread.