Add a Personal Background Picture to Your Gmail

"Gmail themes also did a decent job by providing many great themes one could apply to get rid of the monotonous black and white background, but with the latest integration of the ability to apply custom background as themes it has taken it up a notch and you can now put any picture you want as your Gmail background."

http://www.guidingtech.com/12933/add-personal-background-picture-gmail/

Send Whenever - Schedule Gmail messages

"Schedule emails to send to anyone at any time."

http://sendwhenever.com/

Rapportive

"Rapportive shows you everything about your contacts right inside your inbox.

You can immediately see what people look like, where they're based, and what they do. You can establish rapport by mentioning shared interests. You can grow your network by connecting on LinkedIn, Twitter, Facebook and more. And you can record thoughts for later by leaving notes."

http://rapportive.com/

Rapportive

Rapportive shows you everything about your contacts right inside your inbox.

"You can immediately see what people look like, where they're based, and what they do. You can establish rapport by mentioning shared interests. You can grow your network by connecting on LinkedIn, Twitter, Facebook and more. And you can record thoughts for later by leaving notes."

http://rapportive.com/

http://rapportive.com/twitter

Integrate Facebook, Twitter and Buzz into Your Gmail

“The key was using Gmail Labs feature at the bottom of the list called ‘Add any gadget by URL’. This allowed me to add these 3 features to my Gmail sidebar.”

http://mashable.com/2010/02/12/facebook-twitter-buzz-gmail/

Gmail: How to Allow Another Gmail User Account to Read, Delete, and Send Mail on Your Behalf

"You can now delegate email rights to another Gmail user. This can be useful if you want to give email access to your spouse or business associate and also give them access to your Contacts."

http://www.tech-recipes.com/rx/10217/gmail-how-to-allow-another-gmail-user-account-to-read-delete-and-send-mail-on-your-behalf

Little-known Gmail features

Gmail has a bunch of lesser-known feature that can end up being very useful once you get to know them.

http://gmailblog.blogspot.com/2007/08/top-10-little-known-gmail-features-part.html

20 Ways to Use Gmail Filters

http://www.lifehack.org/articles/technology/20-ways-to-use-gmail-filters.html