I always need this because I need to edit .htaccess files once a while.
For OS X Mavericks 10.9:
defaults write com.apple.finder AppleShowAllFiles TRUE
For Mac OS X 10.8 Mountain Lion, 10.7 Lion, 10.6 Snow Leopard::
defaults write com.apple.Finder AppleShowAllFiles TRUE
Finally, reset Finder and you should see all the files with "." prefix are shown everywhere.
If you want to hide all the hidden files, just change TRUE to FALSE and kill the Finder again.
Alternatively, you can use this widget for fast switching - https://github.com/Deminem/Hidden-Files