Is there a reader for iPhone or iPad that supports many formats? I'm looking for a reader application that can open many ebook formats, specifically I am looking for pdf, djvu, epub, fb2 and txt.
Book Reader Pro is what you are looking for. The ebook file format support is outstanding. The supported file formats are  PDF/EPUB/CHM/DJVU/FB2/CBZ and XPS.

Stanza would be another good choice but I'm not sure about it file format support.