Yahoo! Messenger for Windows Vista
I was just looking at the Yahoo! Messenger for Windows Vista video. Wow. It's gorgeous. It's nice to see people doing amazing things with Avalon/WPF. I was particularly impressed with the great flow and visual as you select the background color (pictured, but watch the video about 2/3rds in for the full effect).
I look forward to more luscious apps like this - looks like I may need to dust off my WPF skills and try making a few things myself (too many projects, too little time).