Gmail is currently facing a worldwide downtime which is affecting both users of Gmail itself and those who make use of the Mail feature in Google Apps. Google has acknowledged the issue and is still working on resolving it, though no wonder how long will it take.
For those looking for a quick solution to send/receive email through their Gmail inbox is to try the Thunderbird Email Client.
According to the post on the official Gmail blog:
If you have IMAP or POP set up already, you should be able to access your mail that way in the meantime.Since Thunderbird automatically configures to run through IMAP, so for now it is the only most recommended email client that will get you running in seconds.
Go download Thunderbird from this link. Once downloaded, select Gmail during the setup wizard, enter your password and you will be able to send/receive email through your Gmail account till the web version is restored. Tested it myself and it works!
The same method can also be used on any other occasion when Gmail is down.
Update: The GMail gadget in iGoogle.com also seems to be working, you can view your mail and compose messages from there.