It is not a real gnome applet per se, but you can use gnubiff, it sits near the clock, in the notification/indicator applet:
gnubiff checks for mail within a file,
a qmail or MH style dir, or an IMAP4
or POP3 or APOP server. It can
display headers (number, sender,
subject, and date) when new mail has
arrived. While gnubiff is
implemented as a GNOME panel applet,
it also runs as an independent icon
on the desktop in other environments.
It is not a real gnome applet per se, but you can use gnubiff, it sits near the clock, in the notification/indicator applet:
Use CloudSN (Cloud Services Notification). It supports gmail, google reader, pop3, imap, twitter, identi.ca
http://chuchiperriman.github.com/cloud-services-notifications/ (PPA available)
The best part is that it supports multiple accounts and notify-osd.