I have been using log4net as my .NET logging tool for ASP.NET web applications for quite some time. I find it extremely easy to use. Actually, it is a no brainer and has yet to fail even once.
Another open source .NET logging library, called NLog, has shown up on SourceForge and claims to be simpler to use than log4net. If you haven't used log4net and fell in love with it, you may want to check out NLog for your .NET logging needs.
Check out NLog here.