The latest version of Gmail Skins an Firefox extension lets you customize Gmail interface. It has a very useful feature — “insert images from Picasa web albums into Gmail” (it does not embed pictures, just puts a liks to the images).
Another interesting feature is that you can insert gadgets from Google Personalized Homepage. The extension shows only the first gadgets from your Personalized Homepage (I can see only the the first two), but in a future version you'll be able to choose the gadgets. Google Reader and Google Calendar gadgets are probably the best choices.
Photos and Pictures, Picasa Album