Chrome: If you like how Apple Mail adds a picture of the sender to messages, or you're just better with faces than names, the Gmail Sender's Picture is a no-brainer. It simply inserts a person's Google Profile pic into your Gmail conversations with them.
That's pretty much all the extension does — install it, refresh Gmail, and anyone who has given themselves a Google Talk, Google Profile or other G-service icon will show up in their messages with you, as well as anyone you've assigned a picture to in your Google Contacts. Google Profile's own project manager chimed in at the extension's comments to register some love for the extension, so maybe we'll see its functionality implemented in Gmail in the future. If not, well, you know where to find this.
Gmail Sender's Picture is a free download, works wherever Chrome does.