If you just want to do a quick-convert, you can actually email the file(s) as an attachment to your free Kindle email address and type CONVERT in the subject field. You will then receive the file via Whispernet on your Kindle in AZW format.
If you absolutely want MOBI, then I agree that Calibre is probably one of the most all-encompassing. It's the only one I've used, and it's fine for most simple PDFs, HTML files, and text files.
If you're converting some really complex PDFs (such as academic research articles), I would hesitate on converting them, as the formatting for footnotes and endnotes, etc. might be ruined. If you're just asking for simple PDFs, though, Calibre's conversion feature or Amazon's conversion feature should work fine.