Ask HN: Where can I get interesting data sets online?

I have recently become interested in data visualisation and the various excellent javascript libraries available for this (e.g. d3.js, dc.js).

I'd like to put together a small portfolio of data visualisation examples. Can HN recommend where I can download interesting data sets to work with? It doesn't really matter what the subject matter is (please interpret "interesting" however you prefer) but it helps if the data has some "meaning". I always find data visualisations most interesting if they help to make a point or support an argument.

Data sets don't need to be huge, 100s or 1000s of records is probably enough. Ideally downloadable in json or csv format.

HN always has something interesting to say so any other comments about the wider subject of data visualisation would be very welcome!