Someone else posted the demo for Sister Jack. Here's the other one that was available on their website a few years ago. Get the final version on Gimme Fiction.
Queen - Feel Like (Under Pressure Demo)
This is probably the gem of the this entire post! Under Pressure has gained a lot more popularity lately (at least here in the States). The finished song is pretty much a demo in itself. The story goes, Queen were writing new songs in the studio one day and David Bowie stopped by while they were working on this cut, tenatively titled Feel Like. David Bowie isn't on this demo, but I'm pretty sure this was recorded the same day (maybe even within hours) as the final version of Under Pressure we here on the Hot Space album and on the radio. Bowie also added his voice to a song called Cool Cat, which is also on Hot Space, but the version with Bowie was released only on some 12" singles.
David Bowie - Space Oddity (Demo)
Here's your David Bowie fix, if the last song left you wanting more. It's just over a minute long, though. The complete song ended up on Hunky Dory.
Queen - I Want It All / Chinese Torture (Demo)
This is kind of a combination demo. Both songs ended up on The Miracle album. Well, the second one was added as a bonus track on the CD.
Foo Fighters - DOA (Demo)
This demo was released on the limited edition Five Songs And A Cover EP which I believe was only at Best Buy. Lots of good songs on there, like a cover of Cream's I Feel Free and a studio version of Skin And Bones.