Sometimes we work hard on a feature, struggle to get it into the product, test it in all possible permutations, ship it...and then forget to tell anyone about it. We seem to have done this with Bulk Edit.
For some entity types (Leads, Opportunities, Accounts, Contacts, Marketing Lists and Cases), you can select a number of records, and on the "More Actions" menu on the grid, click "Edit". You'll be presented with a blank form of that entity type (see below). Any fields you edit on this form will over-write those fields on the selected records. (The UI model is suspiciously similar to the bulk edit functionality in our internal bug tracking software.)
I need to take the blame for the undiscoverability of this feature - I argued for hiding the Edit menu item on the "More Actions" menu rather than an icon on the bar because I didn't want people thinking that clicking the "Edit" icon was the only way to change a record.