The only one I can think of that allows true integration is Outlook with the Chromatic Dragon sync tool for Toodledo.
I use Google contacts online, and Thunderbird offline. Google contacts only works with Toodledo in that you can put the Toodledo iGoogle gadget in your Gmail page if you want, but it is a really good contact manager. Thunderbird has no sync with Toodledo :( , but what I do is use an addon called ThunderBrowse I believe, which sticks a little browser within Thunderbird, so you can have a tab open to Toodledo, and another one for you email reading pane.
Have you tried Gmail with gears? I bet you could use that for your crm without the need for thunderbird.
I have not tried it, and I may, but the problem is I also have a Zimbra mail account, and far less importantly, a hotmail account, and TB can handle all of them. Zimbra also has a desktop app which I have not tried, so I may check that out as well.
My thought is that it would be simpler for TD folks to allow for use of Google contacts than to build their own. Not to mention, for me, administration would be lessened. Just seems a smarter not to "reinvent the wheel" as my co-worker would say.
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