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!
But now, searching for something which could protect my Laptop and let him carry on watching Shaun the sheep, led me on to KeyFreeze.
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/
(If you need something with more options, like blocking selective keys etc.. consider Toddler Keys and Kid-Key-Lock, although I haven't tested them though!)