AbiWord reads its native XML file format, Word 97 files
(this importer is currently being re-written to support more
file format features), RTF, and plain ASCII text.
Projects are underway to create other importers and
exporters--I believe a WordPerfect importer is underway.
You might want to check list archives for previous
discussion of this.
Adding an importer is relatively easy if you have good
documentation on a given file format and are persistent. :)
-- Shaw Terwilliger