Microsoft is ending its adCenter Analytics Beta. The company is no longer accepting new users, although existing users will be able to access the service through the end of 2009. You'll probably want to export your data by December 31st though, unless you're cool with watching it all disappear.
Microsoft adCenter Analytics Beta was a bit of an answer to Google Analytics and other web analytics software although, as the name suggests, it was designed with advertising analytics in mind.
If you were an adCenter Analytics user, or even if you weren't, Microsoft has provided a rather lengthy list of companies offering similar services, including Google Analytics, Omniture, and Yahoo! Web Analytics.