Sorry this took so long (defending a thesis this week).
RZ and SZ are the UNIX versions of zmodem, and also include YMODEM and
XMODEM protocols. Over good lines they work easily 3 times as fast as KERMIT.
Most systems have it, as they are in the public domain and/or are included
in most UNIX packages.
Most shareware comm programs support X Y or Z modem. I like COMMO right
now, as it's small as all get-out, though it seems to be designed with the
programmer in mind, so I don't know if it's right for everyone. I will
probably register my copy if it keeps performing this well.
-Mike