Windows: If you just want to change one file from RTF to TXT format, opening and saving it in your document editor of choice is easy enough. If you've got dozens you want to shift, Doxillon is a useful tool to have around.
Doxillon can convert freely between DOC, DOCX, RTF, text, PDF and HTML files. It can also import WordPerfect WPD files, though it won't convert into them. You can set options to right-click single files within Explorer to launch the program, but it really comes into its own when you have a large number of files you want to convert. Add them, choose an output directory and format, and conversion is a straightforward process.
Doxillion is a free download for Windows only.