Richard J. Fateman must be mentioned for his stimulating scepticism ("Maybe what should be done is [...] to do it with experts instead of physicists-who-can-write-programs.") at the early days of the GiNaC project, for encouragment ("If you are willing to believe that the cost of writing the code is justified by the speed of the code, then you are entitled to do anything, I suppose.") as well as for having formulated the one true driving force behind it all. ("Most of these problems [of system building] arise from the desire to build a nearly-autonomous system for mathematical problem representation and solution: the intent is for the system to make it unnecessary for the user to provide detailed programming at the level of data representation of basic mathematical concepts.")
Masaharu Goto, the author of the Cint C/C++ interpreter did a great job fixing many bugs in Cint to make it work with GiNaC out of the box. It is also relieving that as of version 5.14.39 he changed Cint's license to make it compatible with GPL. Thanks a lot!
Bruno Haible's wonderful GPL'ed library CLN was of invaluable help for our work. In the beginning, we were kluging around with alternative libraries like NTL or GMP, all of which did not quite suite our needs -- till we discovered CLN. His suggestions and fixes of bugs in CLN saved us a lot of time.
Dirk Kreimer's willingness to say goodbye to MapleV was a major boost for GiNaC. He hasn't abandoned Reduce yet, though.
Hubert Spiesberger's endurance while listening to and commenting on our clueless talk in the early days of the project was helpful. He certainly falls in the tough-guy-department.
Richard M. Stallman deserves credit for discouraging comments ("I hope you are not dead set on using C++. C++ is a very badly designed language.") as well as for encouragement ("But happy hacking").
And last but not least, there is Agentur Hummel + Lang, who created the GiNaC-logo for us.