I really enjoyed the much-maligned Hero Builder's Guide, not for any rule/mechanical innovations but because of the way it set the wheels turning in my head for different character concepts.

Unearthed Arcana was great too.  I'd like to see some of the options in there become core rules in 4E.

Quote from: Zachary The FirstI've gotten so much mileage out of that and Wildscape

What am I missing?  Wildscape was the first non-WotC book I picked up.  It looked pretty cool on the shelf, but when I got it home and read it, I didn't actually find anything I wanted to use.  Not trying to crap on it, but I've heard lots of people recommend it, while I'd recommend against it.  So, seriously, what am I missing?
 
 
For me, the most useful non-core book has been Unearthed Arcana.  I really have very few true "house" rules, any more.  UA provided me with most of what I needed.

I'm not a rabid purchaser of RPG books, but I've used two books quite heavily in my own campaign: A Game of Thrones d20 (for combat and social rules), and A Magical Medieval Society (for their society generation and other tidbits).

We're getting a lot of mileage out of the Spell Compendium.

Arcana Evolved. After that probably Iron Heroes.

I have a completely different game that I use for d20... Everway.  By itself it was a pretty useless game, but the cards it came with are great for character building.  Deal out five of them and use the artwork and questions on the back to build a character concept.

But sticking to the original post topic (and the category of the forum in which it was placed)... my favorite and most used d20 (in this case Modern) product I purchased was/is Ronin Art's:

      d20 - Modern - Monstrous Advanced Classes - Vampire.pdf


Made being a vampire in d20 Modern doable.

True20 Adventure Roleplaying - It can beat up DnD and take all it's stuff.:heh:

Quote from: SigmundTrue20 Adventure Roleplaying - It can beat up DnD and take all it's stuff.:heh:

Now that really does follow in the true Gygaxian spirit of D&D!

Toolbox

True20 will probably overtake it eventually, but I have gotten a lot of use out of the tables in Toolbox.  Most other supplements sit on my shelf (or in a box).

Quote from: SobekWhat am I missing?  Wildscape was the first non-WotC book I picked up.  It looked pretty cool on the shelf, but when I got it home and read it, I didn't actually find anything I wanted to use.  Not trying to crap on it, but I've heard lots of people recommend it, while I'd recommend against it.  So, seriously, what am I missing?

Well, for me it really helped flesh out the terrain in my games.  Instead of my players just going down a river, I had an entire section dedicated to helping me describe that river in better, more exciting, concise terms.  In doing so, it also provides DCs, addresses possible challenges or questions that might pop up.  

It also does a great job describing features and possible encounters for every type of terrain mentioned.  It's also great in that it doesn't just give you, say, a "desert", but minor, moderate, and severe versions of a dozen or more types of deserts.  For some folks, that's the sort of thing that comes automatically, but it's always been something I've had to work harder at.  So, for picking it up for $5 during a FFG sale, I'd say it's been worth it.

Quote from: Zachary The FirstWhat 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.

Stone to Steel:  I ran a homebrew and outfitted several of the "civilized" human races in Roman arms and armor, as well as other ancient earth civs.

Conan d20 core book: It's currently my favorite d20 rule set, which is ironic.  I have long bitched about the complexity and bookkeeping required for D&D 3.x.  Conan d20 has a bit more, but the payoff is a pretty darn good evocation of REH's Hyborian world, and to me, it's worth it.

Another vote for Toolbox.  I don't think I've every just opened it and rolled up stuff, but it's been handy to thumb through and send me spinning off on tangents, which is good for creativity.