It's hard to talk about great C&Ds without mentioning Chilling Effects, http://www.chillingeffects.org/notice.cgi, a repository/database of many C&Ds and copyright claims, which receives copies automatically of many letters sent to large websites.

Some of the content in there is particularly golden for humor value (search "perfect 10"), but more importantly, it serves as a first step to quantifying how many of these letters get sent and how many of those are legitimate. Chilling Effects doesn't track what happens afterwards, but it's one step above anecdote, anyway.