This notation (or the vanilla notation, at least) was invented around 1985 in three places independently: by Bruce "Boppo" Tiemann at Caltech, Paul Klimek in Santa Cruz, and Mike Day and Colin Wright in Cambridge. (Hence its occasional British name "Cambridge Notation".) Following this lead, Jack Boyce and Allen Knutson (both also at Caltech) each came up with the juggling state model to explore the phenomenon of excited-state tricks.
Perhaps the first siteswap animator was Allen Knutson's written in 1988, distributed world-wide via the juggling mailing list, and later rec.juggling. Since then a number have been distributed for PC-compatibles, Unix, Macintosh, Amiga...
Bruce Tiemann and Bengt Magnusson have two articles published on the notation: "The Physics Teacher" (November 1989) and "Juggler's World" (Summer 1991). Kendrick "The Edible Dormouse" Zetie has articles in the European juggling magazines Kaskade (issues 37 and 38) and The Catch. There is also the more academic "Juggling Drops and Descents" in the American Mathematical Monthly, January 1994, by Joe Buhler, David Eisenbud, Ron Graham, and Colin Wright.
Glossary of relevant juggling terms