R – What are some great web based interfaces that you use on a day to day basis

Tabs are becoming common place enough that I wouldn't worry about using them, as long as you implement them correctly. Make sure that you make the active tab visually distinct from the other tabs.

Also, try to create the tabs using progressive enhancement so that the content is still there with JavaScript disabled. There are two main ways of doing this:

1. Load every tab but the first using
   AJAX. The tabs themselves should be links to the content that the AJAX
   fetches.
2. Keep all of your information on the page, but hide it using JavaScript. When you cycle through the tabs, they are populated from the hidden parts of the page.

A design resource you might find helpful is the YUI Design Pattern Library and their section on tabs.

For notification, I prefer small notification windows in the bottom right corner. If they have a minimum UI allowing me to take some quick action, it would be ideal.

As model: incoming mail notification from Outlook, Skype notifications for incomming calls, etc.