James:

Let's say you would like to run Y! Mail beta at home, and the classic at work. You may think you'd have to constantly switch from the classic to Y! Mail beta, then switch back again everyday before leaving home.

Not anymore.

When you are at work, change your mail URL from

http://... .mail.yahoo.com/dc/launch? ... 
(URL of Y! Mail beta)

to

http://... .mail.yahoo.com/ym/login?ymv=0

It will let you run Yahoo! Mail classic for the current session only. You can visit http://mail.yahoo.com at any time when you want to run Y! Mail beta again.

So instead of switching back and forth, you can just bookmark the "classic" url and use it at work from now on.

James:

If you have a message opened in it's own tab, you don't have to go back to the message list to move it. Just click on the tab and drag the message into your folder.

James:

There are so many keywords to use for searching messages, it can be hard to remember them all. Never fear, you can view a comprehensive list of search keywords in an integrated pop-up. Just Ctrl+Click on the [Go] button of the message search box.

James:

Have you ever tried accessing Y-Mail beta from a system you knew was compatible, only to be blocked?

I have.

The next time that happens to you, just go to

http://us.mg1.mail.yahoo.com/dc/launch?sysreg=ignore
(you may need to change "us.mg1" depending on your farm)

Y-Mail beta will throw caution to the wind and ignore the system requirements. Use this at your own risk.