There is so many of them
The ones I know are Outlook, Thunderbird, foxmail and dreammail
Most people recommand me Foxmail
When I think Thunderbird is the best(open source)
I previously used Foxmail, but found it a bit difficult with multi-accounts. It had a nice feature where you could send email from any address you like - particularly useful if you don't have access to an SMTP service you need, but this had two major drawbacks:
Firstly, you could only send email to one recepient if you used this feature.
Secondly, more and more email companies are blocking mails from "Direct-Senders" rather than via an authenticated SMTP service.
I use Mozilla Thunderbird - it became significantly easier to use when they upgraded it to version 1.5, now allowing multiple outgoing SMTP servers. As you said, it's open source and there are hundreds of extensions and themes available. It is my firm recommendation.
I use Thunderbird, i'm not sure if is the best, but all the functions that i search in a mail client are in thunderbird and i don't need a big process like outlook running in the CPU
Haven't tried any but these two:
Apple Mail on my Mac-love it
I usually just use web mail - gmail, yahoo mail, squirrel
yeah, mostly web mail for me too, but outlook isn't nearly as bad as everyone here says it is. It ain't great, but it works
I think that hotmail is very good..
I think a stealthy switch to Linux and then Evolution. It's smaller than outlook or Thunderbird but it's just as useful.
I mostly use webmail (GMail) but for the normal mail I'm still using Outlook.
I like the "Agenda" feature and I personally don't like the mozilla mail client (It comes with mozilla, not firfox)