Open Office. One thing I love about using Open Office is the fact that you can easily publish your document or spreadsheet or presentation file to PDF. Just click on "File" and choose "Export as PDF" and your document is ready for sending out!
Microsoft XPS Document Writer. Earlier today I had to scan a ten-page document and since the pages were scanned individually I had no idea how to stitch them together. I poked around the printer settings and wondered what the MS XPS Document Writer was for. So I clicked on all the "pics" and clicked on the print option and tadaaan it stitched together my document. It ate up parts of the document though and I had to find out how to view it. Might get my end-reader confused so I looked for another solution.
Corel Draw 12. One option I use for publishing PDFs is Corel Draw. Good only for one-pagers.
PDF 995. An old friend recommended this solution to me over five years ago and I had it on George and Happy (my old PC and laptop). Just download the installer and after installation it will become a part of your print options. It's ad supported though, but very easy to use. I'm glad it worked on Vista!
Update (thanks for the reminder Yolynne!)
Google Docs. Of course! How could I forget?!?