There are various affiliate networks out there, who offer the promotion of Software downloads. Just to name a few:

1.) www.shareasale.com : you have to pick the single programs and work with the link codes

2.) www.linkshare.com : Apple itunes is using them to promote their MP3 downloads. While that is fine, only a few are downloading the itunes software - that is for bad conversion, IMHO!

3.) www.emusic.com (you can use them via www.cj.com)

4.) www.clickbank.com is very old but very nice. They have their own payment methods and support paypal

5.) www.regnow.com is very old, too. They are better know as Digital River.

These sites are a good place to start!