Here you can find some of the clients of the past. Please note that most of these clients are no longer supported by their authors, and may be very buggy. You are on your own as far as support goes, outside of the Client> forum.
Blackout Group's Client9 - For ISCABBS/DOC; Unix and VMS
Gorby's ISCABBS Client - For ISCABBS; AmigaDOS
Isoroku's ISCABBS Client - For ISCABBS; Unix and VMS
Laughing Kangaroo Client - For ISCABBS; MacOS 68k Systems
PanaSonic's ISCABBS & YAWC BBS clients - For ISCABBS and YAWC; IBM OS/2
Sbum's MacOS Client - For ISCABBS; MacOS PowerPC and 68k Systems