How to comment multiple lines at once?

How can I select a bunch of text and comment it all out? Currently I go to the first line, go to insert mode then type # left-arrowdown-arrow and then I repeat that sequence, perhaps saving a few keystrokes by using the . repeat feature to do each line. Is there anyway I could (for instance) select either multiple lines in visual mode or by using a range of lines and an ex ('colon') command and for that range comment out all the lines with a # to make them a "block comment". The ability to quickly 'de-comment' (remove the #'s) for a block comment would also be nice.