Google has been the most improving mailing web interface that I know. Having a whooping 2 GB storage space when launched, he slapped over the face all his competitors, whom at that time had 4- 5 MB of storage space. Now is 7.5 GB and increasing.
The interface is very clean and intuitive, but not lacking the good looking ( example of bad looking is Squirrel Mail who looks obsolete, due to the right angle corners, opposed to the round corners approach in Gmail). Also the code completion for emails, the spell checker and other goodies makes this interface the most interesting in my opinion.
Recently, I learn about something that is quite hidden in Gmail interface: the plugins.
These can be activated by going into Settings in your Gmail mail account, click on Labs tab and search the desired plugin. In order to add it’s functions on the interface, you must activate first by checking the Enable option. It will be applied as soon as you click ‘Save Changes’ button on the bottom of the page.
I have activated the following plugins:
Give it a try!