As one of a handful of Internet professionals in a PR agency, I’m continually asked for lots of stats to back up proposals as well as using them to support or inform my own recommendations for client work.  Here are several of the best FREE general sources for Internet statistics I’ve found out there:
– A GREAT site updates all the time with new stats and indexed by industry categories.  You’ll find lots of useful stuff here.

Pew Internet
– One of the most credible sources for stats and reports out there … anytime they release a new report, you can bet it will generate media coverage.

ClickZ Statistic Articles – A page listing all recent and archived articles relating to online statistics.  Sometimes they publish new reports here, but often they report on results from other sources.

Nielson Netratings – Most of their stuff is highly valuable and highly expensive, but this link is to their free reports area where a limited subset of their research is available.  It could provide a useful nugget or two.

eMarketer eStat Database – A good database with lots of research reports that generally cost a few hundred dollars.  Using their Browse feature at the bottom of the page lets you drill down on many abstracts, some of which have useful diagrams and data points to support your work.

WebMetro Statistics – Decent collection of stats indexed by article headlines and spanning over several pages

Statistics you won’t find on Ebay – A very useful collection of stats on usage of Ebay, including a list of the most popular listing categories, how many bids items receive on average, and how many auctions are won last minute.  Note, the data is from late 2003 – so it’s a bit out of date but I haven’t seen anything more recent.

Listing of the most viral emails – This page is continually updated with funny viral emails that you might have received over the last few years.  Emails are indexed by category, rating, alphabetically, or by weekly best.  A great place to track buzz around viral emails (as long as they are humorous).

Domain Name Statistics – An authoritative list of worldwide domain name related statistics including how many total domain names are registered, a breakdown of number of IP addresses by country, and registrar stats.