Here is another very good thread I started that is worth adding to this list. It is a huge list of Web Bookmarks, all wrapped up in a single Mobi Ebook. Makes using the Kindle Web Browser MUCH easier. It is not exactly a Kindle Hack/Modification, but I think it should be added to this thread.
Link to the thread:
The Ultimate Kindle Web Bookmarks Mobi Ebook
Link to my Google site, which is essentially the Ebook:
Kindle Web Bookmarks
Link to directly download the latest version of my Mobi Book:
Kindle Web Bookmarks