In OS X Lion and Mountain Lion the default setting is to hide the Library folder, probably to stop people from deleting or damaging files that are necessary for OS X Lion to run properly.

The Library folder was visible by default in past versions of Mac OS X, this is how to get it back:-

Show User ~/Library in OS X Lion & Mountain Lion

Open Terminal, and enter the following command to show or hide the directory:
chflags nohidden ~/Library/

The users Library folder will immediately become visible again. Reverting this back to the standard Lion setting is simple too:

Hide User ~/Library in OS X Lion (default setting)

This returns to the default setting of hiding the user Library directory:
chflags hidden ~/Library

Changes take effect immediately again, and Library becomes invisible to the user.