Grim Tales is basically a complete rewrite of d20 Modern -- it's an all-in-one sourcebook for DMs looking to run a campaign in a wide array of genres, but geared towards low-magic, "Grim" type of stories, where maybe a hero or two goes crazy from the horror, where casting spells saps your health, where the heroes might jump into an armoured vehicle (be that chariot, locomotive or hovercar) to pursue unsavoury bad guys.
It rewrites the fundamentals of the d20 system in such a clear and easy-to-read manner that it's quickly become my preferred reference book for in-play questions on things like AoO, environmental effects and so on. It has detailed material on creating creatures and how to calculate the appropriate CR for them, great Horror and Insanity rules, and the most truly customizable class system imaginable. It's a great hybrid between class-based and point-based chargen.
It's a really great book -- probably the best book I bought last year. Definitely worth the money for any DM who's interested in running non-D&D campaigns.