I just downloaded the Windows version of the freeware...

Yahoo! Zimbra Desktop

...which is the only email client that Yahoo! allows IMAP4 access. I have a paid Yahoo! Plus mail account. But, it only provides POP3 server access which is useful for archiving but useless for leaving and managing mail on Yahoo!'s server. I was amused to find that in a turn of events, the desktop client looks like the Yahoo! Mail web interface.

Zimbra warns you that it may take hours to sync email if you have a lot of mail. I probably have thousands of messages going back to June 2000 in my Yahoo! Mail account. After 20 minutes, it has synced all the way back to May 2008. Yep, this is going to take a long long long while.

One thing that surprised me is that minimizing it on Windows Vista shrinks it down to the right hand status icon tray instead of the left side applications icon list.