DBI - I do a lot of database work, and a lot of times I just need to see if a DB has a table or is available, so I use 'dbish' a lot. It's also a quick way to figure out what DSN's are available on your machine.
HTML::TreeBuilder - once you wrap your brain around this one, you can scrape data from any web page you want. There's also a module HTML::Seamstress which uses this to create a better HTML template system than HTML::Template (IMHO).
Tk - sometimes it's nice to have your favorite script with a GUI interface. :-)
Update: Here's another one:
CGI::Session there's a cookbook and some good tutorials for using this module.
I use several others but these seem to be the most used in my arsenal, and I am a frequent surfer of CPAN, always watching out for anything that makes my job easier.