XKit is described as 'set of tools for Tumblr, designed to make blogging much easier and fun'. There are 2 alternatives to XKit for a variety of platforms, including Mac, Windows, Linux, Google Chrome and Firefox. The best alternative is New XKit, which is both free and Open Source.
A fork (copy with changes) of the original XKit addon, which improves your Tumblr experience. It is comprised with the ability to install more extensions, further adding more features. Please note that the crew behind New XKit is not in contact with the original XKit Guy.