DisplayFusion is "freeware" in the fact that it has a free version and a Pro version. DisplayFusion is generally for multi-monitor setups, but can be used for single monitor setups.
Free version:
Basically, in its simplest form it allows you to have different backgrounds on different monitors (why this has never been native or even built in to video drivers makes me wonder). It will resize the backgrounds based on stretching, tiling etc.
Pro version:
The Pro version adds a lot of value features. For one, desktop backgrounds can rotate randomly between monitors. These can be selected from the same/different files, folders,
Flickr and Vladstudio accounts, etc. There are a lot Flickr Pools for images. There's a couple just for desktop backgrounds as well.
Also, your resizing options are better. It will resize the backgrounds per your preferences on aspect. You can set it to not cut off borders (if needed, because the aspect is different) or so that it won't cut off the borders (though the image may be slightly weird).
You can also hotkey sending windows from one monitor to the next and even have buttons added on their titlebars.
I haven't tried out the "multiple taskbar" feature, where you have your taskbar cloned on the second monitor yet, because it'd probably annoy me.
DisplayFusion is great for those with more than one monitor setup, but not really worth it for single ones, because most of the time you don't pay attention to it.