Google Chrome: We're visual creatures, and sifting through an entire inbox organised only by text isn't exactly pretty. Chrome extension SocialGmail adds contact thumbnails to your inbox, so you can more easily recognise who that new message is from.
It's not that reading the "From" column is particularly hard, but think about it: When you read Twitter or Facebook, does your eye go to the person's name first or their picture? Avatars are really easy ways for our brains to associate a message with a person, and SocialGmail adds that convenience to Gmail with contact pictures next to their name or email address. It's simple, but a nice little extension to add if your inbox is constantly full of messages you want quick access to. Of course, you can also add those pictures to the messages themselves if you so choose.
SocialGmail is a free download, works wherever Google Chrome does.