The Complete Scoundrel
This book is the best one that WOTC has released in ages. Many of their new books over the past couple of years have been rather focused, thus of limited use, and felt like they were padded just to hit a page count.
While Complete Scoundrel may seem targetted to those who play rogues and bards, theres something for everyone here, both players and GMs.
The two big things that are added are Luck Feats and Skill Tricks. While there are plenty of other feats and prestige classes, these two are additions to the game as a whole that everyone can find useful.
Skill Tricks are limited special abilities. They are bought with skill points, and have skill prerequisites. Most of them can only be used once per encounter, unlike feats. I can see adding skill tricks to any d20 campaign and as a GM developing more skill tricks for both D&D and other genres.
Luck Feats give players a second chance if their dice are being treacherous. There are some very interesting abilities among the Luck Feats, and basing it on feats means that characters won't be able to pile up on them too much, and there is a trade off because that means you aren't buying other feats. The only problem with the Luck Feats as a whole is that I didn't see a way to integrate Action Points (used in Eberron and Modern d20). I may have missed it, I scanned through 2 new rulebooks yesterday.
Complete Scoundrel is more for players and DMs that like a role-playing game, as opposed to go in the dungeon, kill the monster, get the loot, rinse and repeat kind of game (though there are some nifty combat abilities). So if you'd rather seduce the daughter of the evil wizard to gain access to his secret tower so that you can steal his plans for the Spell of the Apocalypse, thwart his evil scheme and save the kingdom instead of check for traps-kill 20 orcs-get 4d6 gold and 1 random item from Table 2, this book is for you.
Overall, this book rates 4 of 5 sneaky, lucky, tricky, flying monkeys.
Oh, and you can see my friend Shadowdragon if you want to buy one.