A fast, type-safe object-oriented tween engine for Unity, extensively documented and compatible with all of Unity's scripting languages.
Officially released in the first days of 2012, it has been thorughly tested by numerous users, and is now available on the Asset Store.

HOTween allows you to tween any numeric property or field (including Vectors, Rectangles, etc.), plus some non-numeric ones (like strings). You can create simple tweens or Sequences, which allow you to chain animations like a video timeline. Any tween can simply play, or be controlled like a video player, allowing you to jump at any point, rewind, change speed, and a lot more (see full features).

HOTween can also tween along paths (both curved and linear) using constant speed, and an easy to use and powerful visual editor is available on the website.

HOTween website
Demos
Examples
HOTween open-source code
API docs
Unity Forums Thread