Apple - iTerm as a slide-out terminal from the top of the screen

You can use iTerm2's system-wide hotkey with the Hotkey Window profile to do this.

In iTerm2 preferences, click on the "Keys" tab. In the bottom left, under "Hotkey", check "Show/hide iTerm2 with a system-wide hotkey" and assign the hotkey you'd like to use.

Check the "Hotkey toggles a dedicated window with profile:" option and choose "Hotkey Window" in the popup menu below (should be selected by default).

With default settings, the Hotkey Profile window will stretch across the top of the screen, and the hotkey will drop the window down from the top, complete with animation.

You can customize the settings for the "Hotkey Window" profile under the "Profiles" tab. To make it look like a Quake drop-down terminal, you can use similar "Window" preferences:

If you're not married to iTerm2, you could try TotalTerminal. The slide down window is always a keystroke away.

EDIT - I've loved Total Terminal for a long time. However, in late 2015 I moved to iTerm2. It does all the things I loved TT for, but without any of the issues I ran into.

Just an update on @joelseph answer, with the new interface.