I'll throw in one more vote for java on some sort of unix. It will scale better than the other options and can be done significantly cheaper if that is a concern (jboss + apache + linux = free software on cheap intel hardware). If you go that route definitely checkout Struts as someone else mentioned or Turbine (thir DB layer Torque is awsome to work with).

PHP isn't a bad option either if you need quick development, but in my opinion you suffer latter when you need to update the system as it is harder to write nice structured PHP code.

Personally I woudn't touch .NET or any MS product for a server platform, but I know I am extremely biased and others have already covered most of the reasons why (security, MS license fees, good hardware availablility, etc).

-Mike