According to a stanford.edu research, this is what users do on the Internet:
It was made around year 2000 I think, so it's quite old.
Something more recent(2006) says Internet is mostly used for p0rn: http://www.grupthink.com/topic/433.
More surveys and polls list emailing and shopping on top: http://www.pewinternet.org/trends/Daily_Internet_Activities_1.11.07.htm
Some more stats like usage by continent/countries can be found here: http://www.internetworldstats.com/stats.htm