DocsToGo is pretty popular and usually recommended. It supports native MS Office documents including Word and Excel (I think it supports PowerPoint now too), but sometimes it works faster or more accurately if you allow it to use its automatic conversion system to send files to the Palm in a different format and then convert them back. Of course, to send a native file directly to a TX, you would have to use an expansion card.
Under Windows Mobile 2003 and earlier, Microsoft's own Pocket Word and Pocket Excel were not very popular, and most people don't think they are even as good as the third-party Documents To Go. WM5 is an improvement, but a lot of people would still rather use TextMaker from SoftMaker, if they're lucky enough to have $50, or to catch one of their "crazy prices" sales.