Besides using F1 (which doesn't always work), I did not find an add-in for VS to do this, I have another quick way.
I am using Launchy (from launchy.net) and setting up the Weby plug-in to quickly perform an MSDN search. Therefore, if I search for String.Format, my keyboard sequence (from VS or elsewhere) will be as follows:
alt space msdn String.Format
which leads me to search for MSDN, where usually the first list is what I'm looking for. This is not an ideal solution, but with a fast Internet connection and only a keyboard, it suits me well.
source share