MTW3CValidator

http://brandon.fuller.name/archives/hacks/mtw3cvalidator/

If writing valid markup is important to you—and if it's not it should be—you might be interested in Brandon Fuller's MTW3CValidator. This plug-in will run your pages through the W3C's markup validator as they're published, and can insert an image or custom markup snippet into your pages to indicate whether they are valid or not.

Textile

http://bradchoate.com/mt-plugins/textile

Several different plug-ins are available that add alternate text formatters to Movable Type's standard choices (None and Convert Line Breaks). Brad Choate's Textile plug-in is perhaps the most popular of the bunch. Based on Dean Allen's text-to-HTML conversion tool, the Textile plug-in allows you to use defined text formatting cues in your entries, which are then converted into markup when the entry is published. For example, creating an unordered list with a third-level heading is as simple as this:

h3. Fruit
* Apples
* Oranges
* Bananas

MTAmazon

http://mtamazon.sourceforge.net/

Created by Adam Kalsey and opened up as a SourceForge project, MTAmazon makes it possible to integrate information pulled from Amazon's Web services API into your site's pages. You can use it to insert information about a particular product, the results of a search, products on your Amazon Wish List, and more.