It's super flexible with multiple layout sizes, customizable sidebar and top navigation bar options, as well as a number of font options. Effector also includes an alternate dark version and three basic colour schemes that allow you to tailor it to any palette. And it's totally free.
Complete documentation and setup instructions can be found here.
This free theme is a thank-you for the support you Tumblr users have given us over the years. You can check out the other stuff we're working on at PixelUnion.net.