I've yet not got to the point where I've had to use the more wierd and/or wonderful modules like Parse::RecDescent.
Most of these are probably quite well used by a lot of folk.
In no particular order:

1. CGI
2. Carp
3. MIME::Parser
4. Internet::Mail
5. Net::POP3
6. Exporter
7. Getopt::Std
8. HTML::FormatText
9. HTML::TreeBuilder
10. HTML::Entities

A lot of my first Perl CGIs required these modules, so I've got a soft spot for them :-)

HTML::FormatText is rather funky for hauling plaintext from HTML.