I've seen campuses distribute this software to students. It installs a PDF writer as a virtual printer. At last, Windows users can join the rest of the world in freely creating PDFs. I thought that Foxit creator used to be completely free, but now it's a free one-year trial.
I edited this answer after seeing Maneesh's comment clarifying the question. Here is the old response, which may be useful for others:
It is free and widely available, natively packaged on most popular platforms, including Windows. It is a set of libraries and command-line tools, but GUIs exist and are linked from the ImageMagick main page.
If the content you want to make into PDFs is Office documents, then you can Save As .pdf. In Office 2010, this is built in, but for 2007 you need to install a free add-in
The best choice of tool will depend on what you are starting with. If your files are in a format that ImageMagick supports then use it - it's great. If you want to generate PDF from Microsoft Office, or anything else that ImageMagick doesn't support, then CutePDF should do the trick as long as you have a viewer program for the files that has a Print button.
In my opinion, the best tool is made by yourself. you can customize all the feathers in the pdf conversion tool, like convert pdf to jpg files http://www.xspdf.com/guide/pdf-jpg-converting/ or convert whole pdf to multipage tiff image. so if you are a developer, you can try gostcript or itext to develop a pdf converter yourself. the both tools are free open source, and can be run in modern programming language, such as c# and java.