<p>I'm in the middle of building a pretty big site using asp.net (forms if it's of any interest), and I'm wondering if i need a logging framework such as log4net.
I've used such frameworks before, but i don't see how i could put it to any use in this type of project.
usually i think logging is needed for customer delivered application, where the log could be sent back and then analyzed for errors.
what do you think?</p>

Heck no - logging is
incredibly
important for web applications. If you log appropriately, it makes troubleshooting
so
much easier.
Log4Net is probably a good bet as a framework. You might also want a way of gathering logs together from multiple servers - and even if you don't use more than one server at the moment, it's probably worth suggesting that you will do so at some point.
One option for the centralisation is to copy the logs into a database - see
this question
for more opinions on that front.