Microsoft Excel already can save files as plain text (and you can change the extension to whatever you want afterwards). Why do you need a separate program to do this?
Or you could do it yourself; an xslx is "just" zipped up xml files. You will need to unzip the files and then parse the XML files, rewriting the data to csv format or whatever.
If you want to investigate, your can use any zip utility whch understands the standard zip format to extract the contents of a xslx file. And the XML format is well documented.
PS There is an open source project working on a library, but as they're only at release 0.0.3 is looks like the project is very young. http://libopc.codeplex.com/