What are the best UI design in terms of usability and esthetics you have even seen? I mean both desktop software (of all OS) and website.

My list:

1. Picasa 3 - the way it organizes photos.

2. Find-and-highlight-as-you-type in google Chrome.

3. Dynamic search hints when entering something in the search box in Gmail.

4. I'm not a Mac OS X user, but I have seen in most windows on the top toolbar there are both the icons and texts shown for each function, as apposed to on Windows I have seen many programs (MS Office included) have many small toolbar icons which you can hardly understand what they do until you hover the mouse on it for a while to see the hints (if any).

5. The ability to search an setting in Eclipse IDE.

6. the way to make 3D models in Google Sketchup.

7. the way to label an email in Gmail.

What are you list?

Well, I couldn't resist to list some annoying UI design I have experienced and remember at this moment.

1. IE on Windows server, when you visit the new website, you have to click many times to get it added to the white list before you can start browsing, IIRC, it's not fixed in IE 8 when that last time I used it on Windows 2008.

2. The default search behavior in the File Explorer on Windows xp, that animated thing...

3. the dialog that shows up when you are trying to save a plain text CSV file in Excel after applied some formatting options which does not compatible with CSV.