I've always found it annoying that you can auto-hide the dock on the Mac, but not the menu bar. This new software does just that. Why use it?
Probably because it gives you extra space (it may not look like much, but it matters) and it looks so cool.. so minimalistic. Also if you're like me and don't use the spotlight menu item enough to justify the space it takes up in your menubar, menufela lets you get rid of it.
In addition to autohiding the menu, you can specify a hotkey to toggle the menubar on or off.
Get it at ninja kitten (what a name!).