For better or for worse, there is now commercial software available for Linux. Although it isn't a fancy word processing application, Motif; is a package that must be payed for, and the source isn't given out. Motif is a user interface for The X Window System; that vaguely resembles Microsoft Windows.;
For any readers interested in the legalities of Linux, this is allowed by the license. While the GNU General Public License ;; (reproduced in Appendix A) covers the kernel, the GNU Library General Public License (reproduced in Appendix B) covers most of the computer code applications depend on.
Please note that those two documents are copyright notices, and not licenses to use. They do not regulate how you may use the software, merely under what circumstances you can copy it and any derivative works. Also, copyright notices are enforced by lawsuits by the copyright holders, either the Free Software Foundation or Linus Torvalds. In general, this means you can't go wrong if you obey the spirit of what they're asking--they probably won't sue you and all will be well. (Unless the rights get sold.) It's also a good idea not to think up schemes to get around these two copyrights--it's almost definitely possible, but merely causes grief to all parties involved.