Books you didn't expect much out of -bought anyway- and got a lot of use from.

[Nifft]

Book of Vile Darkness! It's got some great flavorful spells, it's got Shadow Demons, it's got some good details on the Archfiends & Demon-Princes (though I wish there were more of those), and some great mechanics that help make bad guys Evil.

To think, I just bought it for the boobies.

-- N

 

[haiiro]

Most recently, Underdark. I pick up all of the FR books, and all of them have been good so far -- if not always directly useful to me. This one suprised me: even without having any intention of running an Underdark campaign, there's a ton of cool stuff in there that I can use.

I'll chime in on the Fiend Folio as well -- it's a kick-ass monster book, and far more valuable than I ever expected it to be.

 

[demiurge1138]

Yeah, I too bought the Fiend Folio with low expectations (I got it just because I like monsters), and than got a lot of use from it. I'd also nominate Unapproachable East for that honor; I don't even play in the Realms anymore, and I found some great PrCs, spells and monsters there.

Demiurge out.

 

[der_kluge]

I don't really have impulse buys. Well, ok, I did give in and buy the Slayer's Guides to Games Masters recently. Funny product, btw.

I don't think I have any book that I've bought that was just an incidental purchased that I've used heavily.

The books I use the most (Bluffside, Artificer's Handbook) are books I worked on, so those don't really count!

 

[Gnarlo]

For me, it would be the FRCS. I'd left D&D when 2e came out and had missed all of the Realms stuff the first time around. I'd been given the impression by all the things I'd read and discussed about it that it was a campaign for over the top fantasy dual sword wielding munchkins where gods walked the earth and were killed by said munchkins except when they weren't being slapped around and ridiculed by the uber DM pet NPCs built into the setting.

Picked up FRCS after I'd gotten back to D&D with 3e on a whim because it was just. that. pretty., and discovered that it could be all that I had heard about it and a bag of chips, but that it didn't have to be. Been enjoying a more gritty and lower level Realms ever since.

 

[Magius del Cotto]

Mutants and Masterminds all the way. Possibly BESM D20 too (mostly because of the GM). I got M&M just because I could, pretty much, and it turns out that I'm really enjoying playing with it (or I would, if I did't sound like I was going through puberty again, but that's a different story...). I can make any character I want to in under 30 minutes, I've had 3 campaigns with it (two that failed to start, one that seems to be going strong, even though the GM doesn't have much confidence yet, and he's faced with me, the Min/Maxer (who isn't proficient enough with the rules yet to be a threat), and the Munchkin, but anyways...). Definitely the most-used book outside of the core D&D books I have.
Also, the GM for the M&M game just looked through the rules yesterday, and is quite impressed by how easy it is to make a character on the fly - just take a power, add some abilities (or nab them from the front of the book - I need to get the Crooks book so he has more to nab from...), and instant character, ready to go.
So, yeah. Mutants and Masterminds it is.

 

[d20Dwarf]

Then I spotted it on E-Bay for under $10

('Course, I say that about my wife, too....heh heh heh.)

I bid on her too, but you got in that last minute bid due to my dial-up connection at the time.

 

[TheAuldGrump]

The Quintessential Fighter by Mongoose. I bought it because of the Open Mass Combat rules, with very low expectations otherwise, then was amazed with how much I liked the book, the schools of fighting in particular were neat. I have bought nearly all the other Quintessential books since.

The Auld Grump

 

[Mystery Man]

Book of Vile Darkness! It's got some great flavorful spells, it's got Shadow Demons, it's got some good details on the Archfiends & Demon-Princes (though I wish there were more of those), and some great mechanics that help make bad guys Evil.

To think, I just bought it for the boobies.

-- N

Ditto.

 

[MrFilthyIke]

For me the two main entries are Kalamar Campaign setting

I'll agree here, I bought Kalamar because it was just sitting there on a shelf, and because I enjoyed the mini-mod Dan Donnelly(sp?) ran for us at MegaCon 02 (I think).

So, I dropped the money, and have not looked back (now that I have 75% of all the Kalamar products)