What is the most useful d20 book or supplement you've purchased (outside of the Core Books)?  It can be an alternate rules set, source material, setting, whatever.  What book contributed the most to your d20 games, and why?

Oh, and the title should read supplement/book.  Too hasty, far too hasty.

Modern Backdrops.

Of all the books I own (and I don't have many), this location resource has influenced my games more than any.  I honestly can't conceive of a modern adventure that doesn't start out in one of the five cities laid out in the book.  Light on the stats, heavy on the plot hooks and details.

Strangely, Traps & Treachery.  It's got a new NPC class -- the Thug -- a Thievery domain, a few prestige classes, and a few wizard spells.  Somehow, though, every bit of the crunch has been useful to me at one point or another.  

It will soon be surpassed by Iron Heroes, though.  I'm stealing their skill system for everything, including Star Wars.

I have a few answers actually.

Monster Books: Definitely Tome of Horrors 1 - 3. You get most of the monsters from previous editions that they didn't include with the Monster Manual with ToH I. Numbers II and III contain all new material, but they're also very, very good. Also, a bonus for freelancers, they're all 100% open game content.

Alternate Rules: True20. I can't say enough good things about this. I like it because it leaves the stuff I like in D20 while streamlining the rest. Excellent, excellent product if you want to speed up your game play and prep time.

Setting/Source Material: Darwin's World 2nd edition. For me, it has become the definitive post apocalyptic game system ever. Great rules additions and excellent source material for a post apocalyptic game. Keep in mind that I say this having contributed a third of the material to D20 Apocalypse for WotC.

Mine I think so far would have to be Traps & Treachery for all the cool stuff in it.  Also Arms & Armor 3.5 is invaluable just because it gathers stuff fronm like 20+ books.

For me, it would have to be the "Toolbox" supplement. So many wonderful tables...

After that, probably Call of Cthulhu D20.

And third place would be a tie between Fantasy Flight's Cityworks (written by Mike Mearls, if I recall correctly) and Penumbra's Dynasties & Demagogues (that book is pure GOLD for any politically-themed campaign).

RPGPundit

Quote from: RPGPunditFor me, it would have to be the "Toolbox" supplement. So many wonderful tables...

After that, probably Call of Cthulhu D20.

And third place would be a tie between Fantasy Flight's Cityworks (written by Mike Mearls, if I recall correctly) and Penumbra's Dynasties & Demagogues (that book is pure GOLD for any politically-themed campaign).

RPGPundit

I've not tried Dynasties & Demagogues.  My games have really began to have more & more political and diplomatic themes in them--I'll have to give it a look.   Thanks!

Total strangeness--I just put my copy of Cityworks down as I got online.  I've gotten so much mileage out of that and Wildscape--in fact, the whole Legends & Lairs line was pretty great.

It's a tie between BESM d20 and Silver Age Sentinels d20. Together those two gave me the universal d20 rules I'd been looking for with enough compatibility that I could use them with existing material more or less "as is."

Spell Compendium probably.

Two best things about Dynasties and Demagogues:  The rules for the "Beaurocratic maze trap", and the rules for elections/faction voting in parliaments.

Absolute gold.

RPGPundit

Quote from: RPGPunditTwo best things about Dynasties and Demagogues:  The rules for the "Beaurocratic maze trap", and the rules for elections/faction voting in parliaments.

Absolute gold.

RPGPundit

I also think that Dynasties & Demogogues is a tremendous supplement.  Its an extremely useful book if you want to move you game more towards political intrigue and things that the chracaters can't just slaughter.  Lots and lots of great information.

I have probably used the Book of Vile Darkness the most.

But that was also the last book I bought.

I buy less than 1 book a year these days.

Iron Heroes. No question.

Hey, how is Mastering Iron Heroes?  Any good?

I get the most utility out of monster books.  I like chucking new stuff at my players that they have to figure out how to fight properly.  

Beyond that most of RPGObjects' D20 Modern line.  DW2 is a great tool to use with D20 Apocalypse.  D20 Future has also seen a lot of use since I'm working on a big Sci-Fantasy setting.