What are some useful keyboard shortcuts?

You can set up whatever keyboard shortcuts you want through System -> Preferences -> Keyboard Shortcuts. This will also tell you the current shortcuts.

Personally, I activate Ctrl+Alt+Backspace to restart the x server, use Ctrl+Alt+T to open a terminal, Ctrl+Alt+X as the xkill command so I can click on non-responding window to kill it and Ctrl+Alt+Delete to open the system monitor (a la Windows) instead of bringing up the shutdown prompt.

There is also a whole load of keyboard shortcuts set by compiz, which can be edited using compizconfig-settings-manager1 (installable from repositories).

I find keyboard shortcuts to be a personal preference and we are lucky that Linux is so customisable as to allow whatever configuration we want.

There's a fairly substantial list on the Ubuntu community wiki.

https://help.ubuntu.com/community/KeyboardShortcuts

When I found out about Alt + Mouse 1 on an open Window. I went nuts moving windows is easy now.

I also use Ctrl + Alt + (Left|Right) for switching workspaces.

Lastly I find myself using Super + Space which is the default binding for Gnome Do which I love.

Alt + F2 is very handy for starting programs, it has auto-completion and prevents you from clicking though the menus.