One is their annoying vanishing act on some systems- OS X in particular. (It's because of the [noname].[extension] format marking it as a 'system' file, I assume).
Anyway, to show hidden files on Mac OS X Lion 10.7:
Open Terminal (Open Finder, then look in the 'Utilities' folder)
In Terminal, enter this command and press return to run it:
defaults write com.apple.Finder AppleShowAllFiles YES
Relaunch Finder (Alt+Right Click on icon and choose 'relaunch'.).
Hidden files should now appear (as greyed-out icons to show they're hidden). Make sure you don't delete anything important!
To undo this:
defaults write com.apple.Finder AppleShowAllFiles NO
