There are a number of things you can do with these little squares. They’re a very traditional form of crochet (and a really good way of using up odds and ends), you can see the kind of thing I mean if you search for “granny square” on flickr. When I was first learning to crochet I made myself a coding blanket that I still love!
Crochet doesn’t have to be square and it doesn’t have to be traditional – I’ve seen everything from the subversive (crochet covers on parking meters) to the cute (amigurumi). I’m currently working on (currently in the sense that I’ve begun and I haven’t finished yet, rather than it being truly ongoing) a set of hexagonal string coasters. The idea is that they will tesselate and form either a big placemat to put hot pots on or several smaller cup-sized coasters. They’re not radical, but they’re not really your traditional granny square either!
I’m sure there are many more uses of crochet in general and granny squares in particular – answers in the comments please :)