How to access hidden or system files in OS X

Accessing hidden folders from Finder

If you need to go to a system folder not visible through the "normal" Finder, do the following.

Once you have the Finder dialog open, press Cmd+G. Here, enter the path to the folder you want to open.

Accessing hidden files or folders from a dialog

You can show hidden files from any file open dialog by pressing Cmd+Shift+.. Thanks to @kine for mentioning.

If you want to always display hidden files (those starting with a dot or hidden through the extended file attributes), enter the following in a Terminal:

defaults write com.apple.finder AppleShowAllFiles TRUE

Then, reload the Finder by option-clicking its icon and selecting "Relaunch".

You can also toggle this option more easily with an AppleScript. See this post for instructions on how to save the AppleScript as an application that you can use to toggle the option without having to use a terminal.

Directly opening system files through the Terminal

As per Andrew's comment above, of course you can just enter the following:

open -a TextEdit /etc/hosts
open -a TextEdit ~/.somehiddenfile

Or, even shorter:

open -t /etc/hosts

Since:

-t Causes the file to be opened with the default text editor, as determined via LaunchServices

I think the easiest way to toggle the visibility of hidden files is by using TinkerTool. Just check 'Show hidden and system files' and click 'Relaunch Finder'. It's free and also gives you access to a bunch of other useful OS settings.