Internet Explorer 7, although Yahoo says they support "Internet Explorer 5.5 or higher". If you want to use Yahoo Mail Beta in IE7, there's an easy fix: spoofing the user-agent.
First close Internet Explorer. Copy this text to a file ie6.reg :
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\5.0\User Agent]
"Platform"="Windows NT 5.1"
... and double-click on the file to modify the registry. Now you can use Yahoo Mail Beta and other sites that block any browser they didn't hear about, even if that browser name starts with "Internet Explorer".
Test IE7 without installing it
Get Yahoo Mail Beta