Firefox only (Win/Mac/Linux): Firefox extension CheckBoxMate allows you to check/uncheck multiple checkboxes by clicking and dragging with your mouse—perfect for selecting multiple emails in Gmail.
After you've installed the add-on, click a checkbox on a web page and drag to create a rectangle around the checkboxes you want to mark as checked. CheckBoxMate actually toggles the checkboxes, so it will check any checkbox that is unchecked and vice versa. It's a real time saver when you find yourself faced with a lengthy survey form or want to select a block of messages in Gmail.
Even though the extension page says it's Firefox 3.5 compatible, the Firefox add-ons manager sees the maxVersion as 3.0. If you're willing to live on the edge, you can make it work—which did the trick for me. CheckBoxMate is a free download, works anywhere Firefox does.