According to Jan's Wikipedia entry, his main theoretical research programmes are:
- a systematic study of specification methods for abstract data types (starting in 1979, with John V. Tucker);
- the invention, development and application of process algebras, especially ACP (starting in 1984, with Jan Willem Klop, Jos Baeten and others);
- Module Algebra (starting in 1986, together with Paul Klint and Jan Heering);
- Program Algebra (starting in 1998, with Marijke Loots).
Apart from his scientific work, Jan Bergstra has had considerable influence on computer science in the Netherlands via his research and organizational activities. As an example of the impact he has had on CS in the NL, I limit myself to mentioning here that he has supervised over 40 PhD students, many of whom have become academic computer scientists (according to Wikipedia, at least 12 at professorial level).
As a recognition of his work, on May 18 this year, Jan has been inducted into the Royal Netherlands Academy of Arts and Sciences.
I wish Jan many happy returns for his 60th birthday and many more years of seminal scientific activity.
Addendum dated June 23: A special issue of Theoretical Computer Science devoted to the festschrift in honour of Jan Bergstra, edited by I. Bethke, A. Ponse and P.H. Rodenburg, is now available.