I used to use a program called MouseClip, but I cannot find it anymore through Google or Bing.
Are there any alternatives so that my mouse will stay on one monitor? This is really important when playing a game.
Here's a solution for Mac OS X (Full Disclosure: I'm the developer!)
Crossing Options (Toggle independently according to taste):
I actually found an old zip version of MouseClip on an external hard drive. I installed the program and noticed that it is produced by GetHyper. After Googling for that I finally found: http://mc.gethyper.co.uk/
This product seems like it suffers from the ultimate anti-search engine protection.
MouseClip v1.10 here
Home page for the author of mouseclip http://adoxa.110mb.com/
I googled "MouseClip software"
May not be the same software you are talking about, another thing you can check is to set your dual monitors (graphics properties) to extend the desktop instead of duplicate.