calibre supports the conversion of many input formats to many output formats. It can convert every input format in the following list, to every output format.
Input Formats: CBZ, CBR, CBC, CHM, EPUB, FB2, HTML, LIT, LRF, MOBI, ODT, PDF, PRC**, PDB, PML, RB, RTF, SNB, TCR, TXT
Output Formats: EPUB, FB2, OEB, LIT, LRF, MOBI, PDB, PML, RB, PDF, SNB, TCR, TXT
** PRC is a generic format, calibre supports PRC files with TextRead and MOBIBook headers
calibre has much more.