Is there a generally-accepted documentation generator for Objective-C (similar to RDoc for Ruby)? I've seen Doxygen and ObjcDoc, and am wondering which is most widely used.
Doxygen is probably the most widely used option. Because it's not just for ObjC (doxygen supports many other languages), the development is lively and the community quite strong. HeaderDoc (now an open source project), by comparison appears to have largely stagnated. HeaderDoc only produces HTML output, while doxygen also produces PDF, LaTeX and many other output forms besides HTML. Even Apple seems to recommend doxygen, with this guide to automatically producing documentation sets, compatible with Xcode's help viewer, from within your Xcode build process.
It's worth noting that doxygen can read HeaderDoc-style comments, so you can write your documentation in HeaderDoc style and decide later whether to produce the final output using doxygen or HeaderDoc.