On steam, it is possible to purchase a "special edition" of Shadow Warrior (2013).
I've read that this includes an artbook, the soundtrack, and two in-game weapons from other devolver digital titles (the sledgehammer from Serious Sam 3 and a pixelated Hotline Miami katana.
I see that the soundtrack is included, but is anything else included? Or do I have to go to Steam (which I don't really mind).

GOG has a special in-game weapon too - "Includes the GOG.com exclusive classic Shadow Warrior katana!"

edit: Not sure about artbook (probably not included), but this is what you get on GOG:
wallpaper
soundtrack
avatar
Viscera Cleanup Detail: Shadow Warrior
Shadow Warrior katana

The Serious Sam 3 sledgehammer and Hotline Miami katana are most likely Steam exclusive.

Included with the GOG version are: wallpaper+ soundtrack+ avatar+ Viscera Cleanup Detail: Shadow Warrior + Exclusive original Shadow Warrior katana

The Steam "special" edition basically has a digital art book for extra 10 dollars. That's it.

Hotline Miami katana while good in theory in game looks too ugly and blocky to use. If you own Saints Row you can use Penetrator which is enough to detract from authenticity of gameplay. Also wtf BFE hammer, too slow - not katana so it sucks for fans only. Excalibat is for fun and not playable, walking dead weapon either. Both Steam and GOG already have Zilla laser blade but feels too futuristic and kind of light sabre SW. Best weapon for me is classic GOG katana because looks like from original game, match personality of Lo Wang and looks like the one from cover/concept arts. Use default replaced later to Nobitsura Kage if you want get deeper into story because chosen sword is used in all plot related cutscenes even if game states you holding Nobitsura Kage (killing scenes and sword merging looks... interesting to say at least). ;-)

The original Shadow Warrior Katana (GoG exclusive), seems way more badass than those toys at steam. Soundtrack? Check. Artbook? Not check. But here, it is DRM-Free. You buy it, you own it, you don't put third party crap in your machine.

Can someone post a pic of the gog excluse sword? I have game on steam but am curious to see it!