Rui CarmoGoogle Analytics is quite a bit more than your average web statistics application - it's actually geared towards their AdWords service, but the free service can be used to keep track of your web site statistics (a very clever move to collaterally improve indexing for those sites that can't or won't use AdSense...).
A nice alternative for low-volume sites (if you're prepared to maintain it, of course) is Piwik.