Scott Mitchell wrote recently about a plug & play ASP.NET error-logging framework that he and Atif Aziz wrote for an MSDN article a while back. 

The framework is called ELMAH (Error Logging Modules and Handlers) and it's free & open source. Apparently you just install the DLL & add a few lines to your web.config, and it'll start logging errors while allowing administrators to view errors online or even access an RSS feed of recent errors. I usually install a global error handler in my ASP.NET apps & use log4net to log & email the information, but I never put together a web-based error viewer. So if there's a stable framework that wraps all that up, I'm all for it.

You can download & read more about ELMAH here. Some screenshots (courtesy of MSDN):

Viewing the error log

RSS feed of recent errors