Mozjpeg

https://blog.mozilla.org/research/2014/03/05/introducing-the-mozjpeg-project/

Mozilla's building a jpeg encoder which should reduce jpeg size considerably without affecting image quality. Should be interesting to see where they take it.

I'll stick with png whenever I can.

agreed, but most websites (facebook being a prime example) serve jpegs

mozjpeg will convert png to jpeg if it can do so losslessly (which would mean same quality using less disk space)

I wish WebP would take off.

Same. This isn't a competing format though, just an enhanced encoder. I actually don't think firefox supports webp but some pages serve them if you're using Chrome

For me JPEG is obsolete. I prefer PNG most of the time. I really doubt a lossless JPEG would take less space than its PNG counterpart, unless it's not really lossless.

mozjpeg will convert png to jpeg if it can do so losslessly (which would mean same quality using less disk space)

Like others I prefer PNG since it's lossless. Although lossy JPEG does have its uses such as for embedding album art into music files without bloating them.