Installing checkgmail

CheckGmail is a Gnome Panel Applet that checks your gmail account.
To install it, first install all the dependencies:

sudo aptitude install libcompress-zlib-perl libcrypt-blowfish-perl libcrypt-simple-perl libcrypt-ssleay-perl libemail-mime-encodings-perl libfreezethaw-perl libgtk2-trayicon-perl libmd5-perl libxml-libxml-common-perl libxml-libxml-perl libxml-namespacesupport-perl libxml-simple-perl

Then, download the tarball package version of CheckGmail from here. Extract (say to /home/username/Programs/checkgmail) and then add it to your start up programs (System -> Preferences -> Sessions). To run CheckGmail for the first time, run the checkgmail script from the startup directory.

Bonus!
To get cool On-Screen Display notifications of new emails, install gnome-osd:

sudo aptitude install gnome-osd

Then, right click on the CheckGmail icon, click Preferences and in the "Command to excecute on new mail" field enter the following:

gnome-osd-client -f "<message id='gmail'><span foreground='black' background='lightgray'>%m new email(s)</span></message>"

To get your CapsLock/NumLock/ScrollLock light(s) blinking when you get new email install blinkd:

sudo aptitude install blinkd

and then add this to the field above:

;blink -s -r %m

The field should now look like this:

gnome-osd-client -f "<message id='gmail'><span foreground='black' background='lightgray'>%m new email(s)</span></message>";blink -s -r %m

and add this to the "Command to excecute for no mail" field:

blink -s -r 0

This will make the ScrollLock light blink n times, where n is the number of new email messages, and will make it stop blinking when you have read all your mail.

Links of insterest
CheckGmail homepage