Since the license of GNU readline is GPL, there are a number of legal
issues with using it from a commercial application.  I've heard there are
minimalist alternatives out there, but the only one I've been able to find
is libedit from NetBSD:

  http://sourceforge.net/projects/libedit/

Ruby's readline extension doesn't work with libedit, but with a few small
changes, it seems to link, and small test applications seem to work.  The
only problems are that libedit doesn't have HISTORY_STATE, so querying the
history is currently not possible, and that libedit doesn't have
event_hook, so I don't think Ruby threads will play nicely with libedit.

Patches for Ruby and for libedit are available at:

  http://rm-f.net/~cout/ruby/readline/

I make no guarantees about the patches, but they seem to work for me.

Are there any other alternate implementations of readline out there that
might also work?

Paul