I use Thunderbird and have two gmail accounts and my domain based account running through it. Each account stays separate but viewable at the same time. I also run lots of filters.

I also like how Thunderbird will thread my emails so I can see how an email conversation runs (this is especially useful for email groups too).

__________________
Jeanne
NowAssistingYou.com
Virtual Assistant :: Web/Blog Design & Hosting :: Writer