Movable Type already has entry previewing functionality built in but it is extremely restrictive. It only shows you three fields (title, entry body and extended entry) and you have no idea what it will look like on your weblog, amongst your styles and scripts.
Enter LivePreview, a new plugin for Movable Type. With LivePreview, you can preview in the blog context — i.e. see what your entry would look like on your site, styles, scripts et al applied. Other advantages include:
All entry fields are included in the LivePreview, in fact LivePreview is also compatible with the RightFields and CustomFields plugins.
Preview with any template. With LivePreview, you can choose any template to use for the preview, you're not restricted in any way!
Easily switch back and forth between the entry and preview screen.
A demonstration of LivePreview can be watched here.