The Finder's Go > Go to Folder (Shift-Command-G) menu item is a nifty way to access your file system.
First of all, you can use it to see hidden folders, so if you need to view /private/var, you don't have to open a Terminal window to do so.
Secondly, the Go to Folder window is an excellent place to paste a path rather than clicking around to get to a deeply nested folder. I use this pretty often when I'm troubleshooting. After all, if Apple's support documentation tells you to open ~/Library/Mail/V2/MailData to access a file within it, it's very quick to just copy that path and paste it into the window.
To make typing stuff in even faster, the Go to Folder window allows Tab completion (just as Terminal does), and it tries to autofill as you type, as well. So if I type in “/S”…
…and then wait for a moment, my Mac will attempt to figure out my destination and fill it in for me.
I can also start typing a path and hit Tab, and then the most relevant result will fill immediately. So for example, to get the screenshot below, I typed “/S [Tab] L [Tab] cor [Tab].”
Once you get the hang of how Tab completion works, you'll realize it's pretty much the best thing ever, especially for those of us who are less-than-efficient typists. Who, me?
Important to note is that you aren't just limited to using Go to Folder within the Finder itself. The same keyboard shortcut can be used to invoke it within Open/Save dialogs, too.
So you've got a folder buried deep within your file system where you want to keep stuff, huh? Well, this is an easy way to save documents to it. We won't ask why you need to bury things (or why you can't, say, put a shortcut to that folder in your sidebar). That's between you and your Mac, friend.