Unfortunately, quite a few programs stick some data in there for branding or informational purposes. As far as I know, no real filetype is immune. Photoshop sticks monitor inforation and color profiles inside of PNG files when you save them, and Paint.NET sets the software text block in a PNG file to the version of Paint.NET you used. What's worse is it can be difficult to find out too.
When you check the summary tab for files, I believe any of the information below the file group is generated by Explorer on the fly, so the Owner is who's set as the owner via NTFS and the Computer is showing which comptuer the file is currently located on (for network shares).
For Photoshop (CS4 that I tested at least), it depends on the file format you're using. I've confirmed that it'll save your currently loaded color profile to a data chunk in PNG files. While this shouldn't be a huge issue, I guess it could be for those ultra-paranoid. – Joshua – 2009-09-20T17:04:51.213