http://www.web-caching.com/traces-logs.html
most of these are old (95-96) but a couple of them seem good (the CA*netll and the NLANR ones) The NLANR trace is good, I think we can get current traces. However, they are a upper level proxy traces, meaning that
this proxy cache serves the caches of other universities/institutes! See http://www.ircache.net/
This is a paper from UWash "On the scale and performance of cooperative web proxy caching" by Wolman et al. They have traces from UWash and Microsoft from 1999 http://citeseer.nj.nec.com/wolman99scale.html