Here's a mildly amusing challenge and response to that challenge:

(http://www.patrickcraig.co.uk/other/compression.htm)

This page has some nice compression gimmicks (a file that compresses well with one algorithm but hardly at all with another; a file that uncompresses to itself)

(http://www.maximumcompression.com/compression_fun.php)

The large text compression benchmark has some nice finely tuned compression software and statistics.

(http://mattmahoney.net/dc/text.html)

And the Hutter prize is interesting. (Get the 100 Mb file of enwiki8 and the de-compressor to under 16 Mb)

(http://prize.hutter1.net/)