Well, I couldn't find one myself, and can't believe there isn't one (I think). So I decided to make one.
Changelog:
* v1.0.0 - Initial Release
* v1.1.0 - Optimised code greatly, and actually fixed it not changing

* v1.1.1 - Fixed again - this time, it WILL work xD
* v1.1.2 - Removed some useless crap...
* v1.2.0 - Implemented some of the changes that Hawk552 suggested (now you can't say I don't listen)
* v1.2.1 - Optimisation suggested by Hawk552
* v1.2.5 - Added a way to turn the script off with sv_randommap <0/1> (0 for off, 1 for on)
Cvars:
sv_randommap <1/0> - Set to 0 to disable the random map or set to 1 to enable it as normal.
To install:
1: Compile
2: Reference in plugins.ini
3: Start the server up (it will load your startup map then calculate a random map from maps.ini or mapcycle.txt and then change the map)
Suggestions? Problems? Let me know (
IN HERE PLEASE).
For v1.2.5+ please make sure you have written the cvar sv_randommap in server.cfg or this may not work.