Krita have used this to sell their free software on Steam, it's a good way to raise revenue from what I understand, people consider paying for the project on steam as a way of donating toward the project. Krita is available for US$50 on Steam or you can download it for free from their website.
You can also get Blender on Steam for free and there is a companion set of video's you can pay $9.95 for as a way of supporting the project. From what I understand the same videos are available on Blenders website for free.
I also remember seeing a Free Project (I couldn't find it again) which you can install for free but it had donation DLC for 10, 20 and 50 US dollars. The DLC didn't actually contain anything, but you would be recognised as having purchased the DLC which... ummm... might be a good thing for people who don't want to donate anonymously and want a big song and dance instead.
I am really surprised by Gimps absence from Steam Greenlight