Available here (there is also an english version of the application available), this is a Mac OS X / MacOs 8.6-9.x application that converts TXT and most importantly RTF files across several encodings of Greek (Unicode included). The latest update allows for the creation of own customised filters based on XML files. The programme is shareware.
GreekKeysConverter is one of the few (the only one?) tools that processes RTF files, with the immense advantage that only text in Greek is being converted (it operates faster then macro-based utilities).