Development of JETSET, the first member of the "Lund Monte Carlo" family, was begun by members of the Lund theory group in 1978, and has continued since then. A number of people have contributed to this and other programs based on it. The most extensive of these is PYTHIA. Over the years, these two programs more and more came to be maintained in common. In 1997 they were therefore merged to one, under the PYTHIA label. Up until a decade ago the main version was PYTHIA 6.4. After 2004 the evolution slowed down, however, and in 2012 development was definitely stopped. It is now reduced to the status of legacy code.
In 2004 a rewriting from Fortran 77 to C++ was begun, and with the release of PYTHIA 8.1 in 2007 the new code became the official main version. In reality the experimental community continued to rely largely on the 6.4 version for LHC run 1, but by now the transition to PYTHIA 8 is complete. The releases of PYTHIA 8.2 in 2014 and PYTHIA 8.3 in 2019 underline the nature of a by-now mature product with a (reasonably) smooth evolution path.
PYTHIA/JETSET has been in heavy use at almost all high-energy colliders of the last 40+ years, such as the PETRA, PEP, TRISTAN, SLC and LEP e+e- ones, the HERA ep one, the Tevatron pp̅ one and the LHC pp/pA/AA one. It has also been used in the study of all proposed future colliders, in many fixed-target experiments, in cosmic-ray studies, and so on. Its usefulness spans over all stages of a project, from early feasibility studies to detector design to search/analysis strategy development to data interpretation.
The PYTHIA/JETSET code base has also been used by a large number of other programs developed over the years. Some of these have been written by Lund/PYTHIA group members, such as LEPTO, FRITIOF, LUCIAE, ARIADNE, POMPYT, LDC, DIPSY, VINCIA and DIRE. The last two are now integrated into the 8.3 version. The ANGANTYR heavy-ion model was developed as a part of PYTHIA from the onset. An even larger number of programs have been written by non-PYTHIA people, "borrowing" smaller or larger code pieces. Especially the string fragmentation and decay routines are heavily used in most branches of particle physics.
While originally developed only by a few people, today the PYTHIA program is maintained and developed by an international collaboration, listed under Authors.
For a more complete historic overview of the PYTHIA event generator, interested readers are referred to the article "The PYTHIA Event Generator: Past, Present and Future" by Torbjörn Sjöstrand, Comput. Phys. Comm. 246 (2020) 106910 (arXiv:1907.09874v1 [hep-ph]).
Similarly the PYTHIA code is intended to provide you with answers to many questions you may have about high-energy collisions, but it is then up to you to use sane judgement when you interpret these answers.