Windows, Mac and Linux: PDFs are great, and while many ereaders do support them they're just not optimised for viewing. There are also plenty of apps that convert PDF to HTML to help solve this problem, but that tends to destroy the great formatting you get with a PDF. Basically, you always have to sacrifice something. With PdfMasher, you don't.
PdfMasher lets you import a PDF and turn it into an HTML document that actually doesn't suck. When you're making the conversion you select and identify certain text blocks so it knows what's what. That way it won't just throw text anywhere, but treat it like the kind of text it is. You can also ignore irrelevant pages, link footnotes, and more. While PdfMasher is in early development and still has a ways to go, it's shaping up to be a really handy ebook conversion tool. If you want it now, it's available for free for Windows, Mac and Linux (although donations are appreciated).