EDIT: Since the OP is a wiki, feel free to add the DLC to the list, just follow the order and formatting I used. I am working all day so if it isn't done by the end of the day I will do it then.
Alright got most of the big Steam titles sans DLC updated in the OP with vouchers reflected in the pricing.
20% off Digital Downloads with voucher code:
GMG20-FXYD1-7WHP1
Offer Ends Friday, October 26, 2012 at 7:00am Eastern
Horror Deals (up to 75% off and stack with above voucher):
Full list of deals - Check the list for DLC on Steamworks games
Alan Wake $11.97 (Steamworks) with above voucher code
Alan Wake: Collector's Edition $13.97 (Steamworks) with above voucher code
Alan Wake's American Nightmare $5.97 (Steamworks) with above voucher code
All Zombies Must Die $3.20 (Steamworks) with above voucher code
The Darkness II $5.99 (Steamworks) with above voucher code
Darksiders $4 (Steamworks) with above voucher code
Darksiders 2 $19.99 (Steamworks) with above voucher code
Dead Island: GOTY $7.99 (Steamworks) with above voucher code
F.E.A.R. 3 $7.99 (Steamworks) with above voucher code
Killing Floor $7.99 (Steamworks) with above voucher code
Metro 2033 $4 (Steamworks) with above voucher code
Risen 2: Dark Waters $13.59 (Steamworks) with above voucher code