Okular (KDE4) has rudimentry support for EPUB.
Remember that Calibre includes an ebook reader (as well as conversion tools).
Can you more specific about converters? Do you want conversion *to* EPUB and PDF, or *from* EPUB and PDF? Either way, what are the other formats you have in mind? E.g. do you just mean website->epub and website->pdf converters?
There's EPUBReader as well, which is a firefox plugin.
Oh, and CoolReader. You might have to install that from source though.