Ecco Pro

Ecco Pro had a cult following. Unfortunately the software was sold a couple times and ended up at Net Manage, who killed it (along with a few others ).

http://www.google.com/search?num=20&hl=en&q=%22ecco+pro%22

But you can download an old version for free, which runs on Win32 systems. Here's one article about it.

http://www.eccorocks.com/

converting to MsOutlook http://support.microsoft.com/?kbid=164046

Sept'2007: Scott Rosenberg notes someone building around it again, with Lua.