[: GaB :]

I always prefer https over plain http, and there is a new feature in gmail to force it to always use an encrypted protocol. Read more here:

http://gmailblog.blogspot.com/2008/07/maki...

I also found two uesful "labs" options:

• Fixed width font: it adds an option for any message to view it in a fixed width font.
• Signature tweaks: finally when clicking reply, my signature is placed above and not below the quoted message.

Many Linux distributions allow you to select a default e-mail application to use. You should have the option to specify a custom one.
For GMail users it is convenient that clicking on an e-mail field, GMail and not, say Evolution pops up.

This small script can be used to achieve this:

#!/bin/sh
 
if [ "$#" -eq 1 ]; then
    suffix="/mail?view=cm&tf=0&to="`echo $1 | sed 's/mailto://'`
fi
 
firefox -remote "openurl(https://mail.google.com$suffix,new-tab)"

If an argument is present, (after trimming mailto:) it treats it as an addressee, and opens a compose page. Otherwise it opens the default inbox page. Pages are opened in a new tab and the script returns immediately.

I'm using GMail, actually (one of) the best webmail services around. But it lacks the capability to encrypt and sign your mail or verify the signature and decrypt your incoming mail.
Although there are solutions on the web, none of them is perfect. I looked primary for Firefox plugins.

The first hit looks great, but should not be used. It is technically a RSA encryption written in JavaScript. Because of this, it works with keys with lengths I can easily crack on my mobile phone.
http://www.langenhoven.com/code/emailencry...
http://www.langenhoven.com/code/encryptthi...

The next one is more impressive: it is called FireGPG. GPG is the Gnu version of PGP (Pretty Good Privacy), that again is essentially the brainchild of one person, Phil Zimmermann, whose motto is: "If privacy is outlawed, only outlaws will have privacy." Long story short, he had a controversy with the US legal system because the fact he published his PGP implementation.
FireGPG isn't a key manager, you must install the GnuPG software yourself. But after that, you will be able to encrypt/sign composed messages in one click, and decrypt/verify incoming messages even more faster. It integrates itself not just into GMail, but will be also available in Firefox's context menus, this way you can use it's features on every input field. It allows, but can't encrypt/decrypt/sign/verify attachments. I also ran into bugs, where some special characters in the body messed up the validity of a signature.
http://firegpg.tuxfamily.org/

The last one is a Firefox plugin bringing S/MIME capabilities to GMail. S/MIME is a standard  of encrypting/decrypting e-mails (including attachments) with X.509 certificates (TLS/HTTPS uses it too). The standard itself is great, but the implementation itself is still beta (in my eyes). At the time it can't handle multiple recipients, if it detects that you own a certificate of the recipient it can't be disabled for a single message, it disables the auto-save feature of GMail (that is a good idea for security, but no so good for usability). But if the development makes progress, it could be a promising project.
http://richard.jones.name/google-hacks/gma...