If you have multiple input languages installed in Windows (under the Region and Language control panel item → Keyboards and Languages), is there a quick way to switch between languages (e.g. switch between English and Russian keyboards)?
The default shortcut to rotate languages is Left Alt+Shift.
If you have more than two languages, I suggest you to set separate shortcuts for the different languages in Language preferences → Advanced settings → Change language bar hot keys
For example, I have Ctrl+Shift+1 set to Russian and Ctrl+Shift+2 set to Japanese IME, as I have 8 more layouts installed and it would take way too long to rotate through them all.
Update Aug 17: As of Windows 10, there is also the new Win + Space shortcut available.
The default shortcut key to switch between input languages is Left Alt+Shift in Windows (tested on Windows 7).