Pretty impressive third-party UI replacement for Windows Mobile. Not only is it impressive, but it is free too.

PointUI

Home's intuitive controls and highly responsive user interface makes using your device a breeze. Here are a few of the key features Home has to offer:

• Control your device via touch or your thumb with the directional control
• Smart Lists Navigation ™ with bookmarks allows you to easily locate an item in even the largest lists
• Choose the slide navigation that suits you
• One touch access to calls, email, text messages and music
• Easily see who and when calls were received, made and missed
• Using the Applet Ribbon ™ you can easily access the time, tasks, appointments, calendar and up to date weather forecasts and current temperatures
• Easily manage your sound, wireless, battery and device settings
• Easy create new emails, text messages, multimedia messages, tasks, appointments, documents and spreadsheets
• Load and switch between running applications effortlessly