Unity has established itself as the default middleware for indie developers these days (with Unreal Engine snapping at its heels), but that doesn't mean there aren't extremely capable alternatives kicking about. One such alternative is GameMaker: Studio, which usually retails at $US50 for the Standard edition, but is currently free until 2 March.
Even if you don't have any immediate aspirations to develop your own game, it can't hurt to have a copy available for a creative rainy day. All you have to do to cash in is download the free version and install it, after which you'll be provided instructions to activate the Standard edition.
If you already have GameMaker: Studio Standard, or want to go all out while the deal is available, you can update to Professional for $US35, a discount on the usual $US99.99 price tag.