Invaluable Rated 5 out of 5 stars
I've been using Missing e for a couple months now, and it has always been a wonderful addition to my tumblr experience.
You should check the website/blog for this if you are concerned about tumblr's dislike of the functionality of this, because Jeremy is honest and clear about what this extension does and how it functions. Don't be frightened by whatever tumblr states (like in their pop-up campaign of late December 2011), just be sure that you disable the add-on before contacting tumblr support about any issues.
I think the best part of Missing e is that the settings pages give you complete control over which of the many features you want to use. That flexibility and the diversity of options at your hands makes this an invaluable addition to your tumblr experience. It's a very straightforward program, and it's even easy to style custom user themes (or 'userstyles') with.
I'm going to be using this extension until tumblr makes all of its features a part of the site.