An alternative to reCAPTCHA...still not perfect. It could trip up someone with cognitive disabilities..or pretty much anyone having an off day. But perhaps it's better than asking a blind user to remember an 8 digit sequence with JAWS talking over the recording and having to repeatedly enter/exit forms mode? It could be edited to include an easier sequence which would make it even more accessible for everyone except spambots.
The benefit of Meebo is that it is chat network agnostic and it allows the person requesting help to not have a chat application/account somewhere. To be a replacement, a service would at least need to fill those two needs.