
iGetcha.org is a suggested opensource and free software directory for the technological sustainability of nonprofits. It's also a simple, non-techie shortlist to assist organizations with a SOHO.
Thunderbird is one of the best free email client out there for nonprofits. If you're not using webmail and need something free, this is the email client to use. Our main idea is, use Thunderbird to limit your connection time online. In less developed countries, this is key to saving on cost for Internet packages that charge per amount of data downloaded (Kbytes). Even if you are not worried about that, Thunderbird is the best option for a nonprofit if you can't afford an operating system package with a topnotch email client.
Thanks to Optimus Media for the tutorial above! It was the best video we could find, but we recommend IMAP over POP because your email client will actually sync up with your email account such as Gmail. If you use POP, it will download everything as new if you use multiple clients. For instance, if you use a client on your smartphone, you wouldn't want your Thunderbird client at home to show messages as not having been open if you already opened them on your smartphone ;)

One other cool thing is, Thunderbird 3 with IMAP is easy to set up because it launches with a wizard setup. And with IMAP, an email client syncs with a server and checks if a message has been accessed. So, if you open Thunderbird on a different computer, you will clearly see which messages are new! When compared to POP, this is really awesome because using an email client with POP, downloads all messages on a server marked as new messages, which can be confusing.