ps2pdf ~ This command comes with most Linux distributions, so it's free. After you've printed a PostScript (ps) file from any Linux program, just run the following command:

ps2pdf original_file.ps new_file.pdf

Works like a charm!

Ghostscript ~ Ghostscript is a set of free programs for Unix and Windows that will convert PostScript and other files to PDF. You can do a lot with Ghostscript, so be prepared to read all of the extensive documentation. Download and read everything at http://www.cs.wisc.edu/~ghost/index.htm