In the spirit of this question...
Is there an indicator applet that tells me when I've received an email through any IMAP service? If so, where can I get it?
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.
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.