If you are wanting to use <alt>-<space> as the key for the excellent Ulauncher with Ubuntu, there are a couple of tweaks that you need to do:

1. Ensure that CompizConfig Settings Manager is installed:

apt-get install compizconfig-settings-manager

2. Start the CompizConfig Settings Manager

Go to General, General options, Key bindings and disable Window Menu which is assigned <alt>-<space> by default.

Go to the Ubuntu Unity Plugin and disable Key to show the menu bar while pressed which is assigned to <alt>.

3. Create a new desktop file for ulauncher called

~/.config/autostart/ulauncher.desktop

with these contents:

[Desktop Entry]
Name=Ulauncher
Comment=Provides a convenient and fast way to launch your desktop applications
Categories=GNOME;Utility;
Exec=ulauncher --hide-window
Icon=ulauncher
Terminal=false
Type=Application
X-GNOME-Autostart-enabled=true
X-GNOME-Autostart-Delay=5

The key part here is the autostart delay, which means that ulauncher starts a little later than other things, and the <alt>-<space> assignment works correctly.

4. Start Ulauncher and assign <alt>-<space> as the hotkey in the Ulauncher Hotkey setting.

5. Close the settings and logout and re-login and <alt>-<space> should now work to start Ulauncher, and away you go.