When visiting a vendor, by default your backpack will open. With this AddOn installed, all of your bags will be opened for you automatically. Small, simple - but useful if you're too lazy to hit Shift-B all the time like I am :)
To install, simply extract to your top-level WoW directory.
Now also works at banks and mailboxes.
/vb off - disable vendor bags
/vb on - re-enable vendor bags
/vb bank - toggle opening & closing at banks (default: on)
/vb bankBags - toggle opening of bank bags (default: on)
/vb mail - toggle opening & closing at mailboxes (default: off)
/vb show - display current settings