I have a Yahoo mail account but rarely use it; Gmail has become my personal email service of choice. 

Recently, I happened to log in and try their latest beta, which is impressively full of second generation AJAX goodness. It leapfrogs Gmail in several respects in terms of usability– it has a familiar Outlook/Thunderbird style UI, with a preview pane plus tabs for viewing messages, and multiple select and familiar keyboard shortcuts for message manipulation. 

The three pane layout will be instantly familiar to anyone who uses Outlook; even the keyboard shortcuts work exactly as expected.  You can shift or control-click to select multiple items, hit delete or CTRL-D to delete messages, drag and drop items to file them, and so forth.   The UI is polished and responsive enough that many times you’ll forget you’re running in web app.

Most impressively, even the scrollbar works exactly as you’d expect– rather than having to page through your inbox or search results, you can use the scrollbar or mousewheel to get to any message in your inbox quickly.  Yahoo mail also includes a fast Ajax search functionality.

Tab for viewing messages are one of those innovations that are so obvious in retrospect you wonder why they haven’t been done before. You can single click on a message to view it in a preview pane or double click to see it in a new tab (not a browser tab, but an application “tab” that works essentially the same way.)  This provides a fast and obvious way to navigate between messages. 

I’m not planning to abandon my gmail account just yet, but I do hope that Google will catch up with some of these features, particularly the tabs and infinite scrollbar.