Data sets to 'practise' with while studying machine learning?

I took a machine learning course this year which covered a lot of different algorithms and techniques, but only involved coursework to implement one or two of them. I think it would be helpful to implement some more, and to a greater variety of data sets, to reinforce my understanding of the algorithms and the differences between them.

Is anyone aware of any good data sets freely available online that would be good for this kind of thing?