WordPress.org

Automatically format titles across your website include the browser title, on posts and pages, category and tag lists as well as in feeds.

This plugin catches common title case formatting mistakes by capitalizing each work in a title. As of version 1.5, users can also select a minimum letter count to capitalize (by default two letter words and smaller are not capitalized).

For example:

"the quick brown fox" will appear as "The Quick Brown Fox"

"i like wordpress so much" will appear as "I Like WordPress so Much"

To learn more, visit http://thisismyurl.com/downloads/wordpress-plugins/wp-title-case/