Does anyone have information about how much money Valve takes off of every steam sale made? Like apple takes 30% of everything that is bought on iTunes. Would be glad if someone could provide a link. I'm buying heavily on steam and I would like to see how I can let the developers profit the best.

No one knows such a thing besides valve. They won't even disclose how many copies they sold for online distribution.

This would be interesting to find out.

what I do know is that when some guy from gearbox said that they were exploiting indie developers, a bunch of those guys defended valve so I don't think they take too much.
 
 http://www.rockpapershotgun.com/2009/10/12/the-steamy-issue-of-digital-distribution/
 
here's the article if you wanna read

The exact numbers are not avaliable for the public, Tripwire has though commented on the rates before.

"Without breaking any non-disclosure agreements, let me just say that our royalty deal was great, and is in line with what I understand that other digital distribution services are offering. We were able to recoup our development costs for our first game within the first week of sales, and sales were straight profit from that point on."

Whew. I was worried my pristine image of Valve and it's suspiciously god-like services might be sullied.

Obviously I'm speculating, but I would imagine after you remove manufacturing costs, distribution costs, retail's cut, etc. whatever Steam takes will seem pretty reasonable.

Pitchford makes good points how a developer and a distribution center being the same company can be a problem. Valve does make it known that their games have never gone without a front page showing. And it does make sense for Pitchford to be worried about small developers because they do not pull in the majority of the money. But steam makes a very good effort to make people at least look at indie games. Just look how many free weekends killing floor has had.

30% seems about right, I've heard from multiple developers that's what Microsoft takes off the top of Xbox Live Marketplace sales.

HAHAHAAHA, Valve isn't in it to make money. They're clearly run by crazy people...awesome crazy people judging by the sales they have. 
 
But in all seriousness I think 20ish% is about right.

Regardless of the percent they get. Say valve and a different developer make two games that would be considered rivals to each other. Say both games are to be released on steam. Which one do you think will see more front page coverage on steam? 

I think that is Randy Pitchford's concern.