http://www.online-convert.com/ has a whole passel of free conversions available including, as you can see, conversions for ebooks.
Download Squad was suspicious of it, but tested it out and was pleased with the results.
I also tried converting a Docx document to PDF. The PDF converter lets you specify a “target device” so that, for example, you can produce PDFs that are specifically aimed at the Kindle. It didn’t work well with my document because it was right-to-left (Hebrew) text, but it did dish out a valid (if left-aligned) PDF.
They also tried it out with an MP3 and liked all the options that the site offered. If anyone uses it please let us know how it worked out.
There is also another one called Online-Converter at http://www.online-converter.com. It offers a different mix (somewhat) and includes units of measure. I haven’t tried either service so have no idea how well either works.