To show all files enter:

defaults write com.apple.finder AppleShowAllFiles 1 && killall Finder

To hide hidden files again

defaults write com.apple.finder AppleShowAllFiles 0 && killall Finder

1 = show all files

0 = do not show all files

In OS X Mountain Lion / Mavericks, you can no longer open hidden folders normally (by double-clicking).

You can however, open hidden folders by right-clicking on them, and clicking “Open”.

On previous versions of mac os:

To show all files enter:

defaults write com.apple.finder AppleShowAllFiles TRUE && killall Finder