Google just rolled out a new feature in Gmail -- Themes. This has been talked about for quite some time, and you can tell they have been working hard on it. The themes available to be installed on your account are truly beautiful, and I'm positive this have a significant impact on both user retention and even new users.
There are tons of themes to choose from -- they range from a theme that looks like you are using a terminal application to ones that change throughout the day like some available for iGoogle.
There is currently no way to create your own themes (without using something like Greasemonkey), but since they are allowing people to create iGoogle themes now, I don't see any reason why they can't do the same with Gmail.
To add themes to your Gmail, click on "settings" in the top right corner, then select the "Themes" tab. From there, you can simply click the themes you want to try out, and you're done.
What do you think of the new themes?