In Mac OS X 10.7 (Lion), Apple decided to make it more difficult to access your Library folder, presumably to stop people doing stuff that made apps crash. But as a developer, I need access to my Library folder a lot. I need to check that preferences are being saved correctly. If I have an app that uses the Application Support folder, then I need to be able to check it. And for sand-boxed apps, they keep all their data in the Containers folder inside the Library.
You can easily get to the Library folder by holding down the Option key while choosing the Go menu in Finder. This adds Library to the menu and you can get to the folder that way. But this is not as convenient as single-click access from a Finder window, so here is my preferred method.
Use the Option key and the Finder’s Go menu to get the Library folder open in a Finder window. Switch this window to column view, which will display the Library folder (slightly greyed out) in the first column. Drag this Library folder to the side bar of your Finder window. Now it is there all the time, in every Finder window that is showing the side bar.
I have read about various Terminal tricks to get the Library to show up, but every system update seems to turn it off again. This technique doesn’t involve anything except the Finder’s side bar preferences and so far (I’m now up to 10.7.4), it hasn’t needed to be reset after any update.