TotalTerminal is a plugin for OS X's native terminal that allows quick access by a hotkey (such as Ctrl+~). For other plugins see here.
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Pro Allows quick access to the terminal system-wide
TotalTerminal is a plugin for Terminal.app. It provides persistent Visor Window which slides down when you press a hot-key.
Pro Quake style window show/hide
You maybe know or don't know Quake from Id Software. But this game had the best idea to hide/show a terminal window. Simply using full width but 26 rows. On show the widow slid down from top of screen. On hide it mode up to top edge of the screen until it disappeared. 2nd next best was the semi translucent background. You could still see through the terminal window while it was shown.
The same style is now available in TotalTerminal and iTerm2 (OSX Terminal replacement - no Terminal plugin).
Con No longer under active development
Con Doesn't work on El Capitan with SIP on
Apple introduced a new security feature in OSX El Capitan, called System Integrity Protection, that, among other things, does not allow augmenting Terminal. While it's possible to turn SIP off (instructions on how to turn off SIP and get TotalTerminal working on El Capitan can be found here) it is not recommended.