orcishgamer: Actually Amazon.com is pretty good about honoring those sorts of things. Some of their promo codes are good for all of January so you can score anything that goes on sale during that time.
I've very much gotten nearly free stuff off of Amazon.com with my codes. I got an XBox 360 Kinect Bundle on launch week for something like 70 bucks I had so many codes built up.
EDIT: I should say I've gotten a lot of free digital music with codes in the past too.
nijuu: So current promo codes will be stackable?
Different codes have different rules. I think the digital download ones generally are not. Some are even one per purchase, but as long as you don't mind doing a checkout per item (which Amazon won't prevent you from doing) this is no issue.
Stackable codes tend to be the ones you get from buying pre-release video games (you get the code the same day it ships). It used to be pretty easy to score 20 USD codes from pre-orders (now the best is normally 10 USD) so things are definitely not as awesome as they once were. They'll have sent you an email for each code, either he email will contain the rules for use or will have a link to them. Check what they stipulate.
I ended up with about 10x$5 codes from their RPG download sale a few days ago so I have some shopping to do as well.
Careful, I noticed some codes are only good between Jan 1 and Jan 11 while others went through the 31st.