Sometimes, it is usefull to see hidden files in Finder. It can be achieved by issuing the following command in a shell:

defaults write com.apple.finder AppleShowAllFiles TRUE

Finder has to be restarted in order for the new setting to take effect.

To hide the hidden files again, relace the word TRUE by FALSE in the command above.

UPDATE (2010-01-02): I found a very handy little widget to toggle between showing and not showing hidden files in Finder. Here is the link: http://www.apple.com/downloads/dashboard/developer/hiddenfiles.html