Necessity is, quite rightly, the mother of all inventions (and… discoveries!).
Before my son came along, I never felt the need to even search or test for this kind of software!
This is a great application which..
“blocks your keyboard and mouse without "locking" the screen. So your kids can safely watch a cartoon or have a videochat with their grandparents and bang the keyboard all they want.”
And not only children, this can be used for pesky pets too (who love toeing on your keyboard) !
Moreover, like all good things, this is Free!
(Note: Needs .net framework to run!)
Checkout KeyFreeze at http://keyfreeze.com/