Heckelphonists (present and past):

• Stella Amar (Canada) - oboist, plays heckelphone #4054 [♯]
• Samuel Andreyev (1981-; Strasbourg, France) - oboist, composer and poet; plays heckelphone #19 (made in 1906) [♯]
• Samuel Bastos (1987-2019; Switzerland) - oboist, used to play #4988
• Cornelia Biggers (1935-2020; USA) - bassoonist; used to play heckelphone #4963 (made in 1957)
• Albrecht Bode (1942-; Köln, Germany) - oboist (and organ builder); plays heckelphone since 2005 [♯]
• Ulrich Brokamp (1957-; Düsseldorf, Germany) - oboist; plays heckelphone #4918 (made in 1956) since 1987 [♯]
• Gerald Corey (1934-2010; Ottawa, Canada) - bassonist and co-founder of the International Double Reed Society (IDRS)
• Margaret Cookhorn (Birmingham, UK) - bassoonist [♯]
• Markus Deuter (1961-; Wien, Austria) - oboist [♯]
• Robert Dewar (1945-2015; New York City, NY, USA) - bassoonist and computer scientist
• Frederic (Fred) Dutton (1928-; Reseda, CA, USA) - bassoonist and bass player, former member of the Dave Brubeck quartet, used to play heckelphone #3282
• John Ellis (1943-2015) - oboist and oenologist; played heckelphone #33
• Bernhard Emmerling (1951-; Oberhaching, Germany) - oboist [♯]
• Carl Erkert (1873-??; Köln, Germany) - oboist; played the heckelphone at its first formal demonstration, on 11 August 1904
• Margaret Friederich (1960-; Heidelberg, Germany) - oboist and specialist for rare woodwind instruments; plays heckelphone #3917 (made in 1925) [♯]
• Alain Girard (Biel, Switzerland) - oboist [♯]
• Ross Gorman (1890-1953) - clarinettist, saxophonist and bandleader
• Arthur Grossman (Seattle, WA, USA) - bassoonist; plays heckelphone since 2000 [♯]
• Theo Heinrichs (Mainz, Germany) - oboist
• Alfred Hertel (1935-2018; Austria) - oboist
• Thomas Hiniker (Rochester, MN, USA) - oboist and oboe maker [♯]
• Robert Howe (1956-; Wilbraham, MA, USA) - oboist, collector and physician [♯]
• Dale Hunter (Montréal and Toronto, Canada) - oboist [♯]
• Peter Hurd (Bellingham, WA, USA) - oboist and collector [♯]
• Gunther Joppig (1943-; Munich, Germany) - oboist, musicologist and cabinet maker [♯]
• Thomas Kiefer (1954-; Gelsenkirchen, Germany) - bassoonist [♯]
• Georg Otto Klapproth (1936-; Köln, Germany) - oboist
• Norman Kuhnert (1965-; Chemnitz, Germany) - bassoonist; plays heckelphone since 2018 [♯]
• Lajos Lencsés (1943-; Stuttgart, Germany) - oboist; played heckelphone in a well-known recording of the Hindemith Trio [♯]
• Donald MacCourt (Ridgewood, NJ, USA) - bassoonist; used to play heckelphones #4981, #4014
• John McDougall (London, UK) - bassoonist; plays heckelphone #4014 (made in 1927) [♯]
• Georg Meerwein (1932-2016; Bamberg, Germany) - oboist
• John Orford (UK) - bassoonist, plays heckelphone since 1981, plays heckelphone #4992 (made in 1990) [♯]
• Mark Perchanok (New York City, NY, USA) - oboist; plays heckelphone #4773 (made in 1950), e.g., in recordings with the Paul Winter Consort [♯]
• Winfried Petri (1914-2000; Germany) - bassonist, used to play heckelphone #4982
• Wolfgang Piesk (1952-; Sauerlach, Germany) - bassoonist, plays heckelphone #4982 [♯]
• Wolfgang Plank (1971-; Vienna, Austria) - oboist; plays heckelphone #42 (made in 1909) [♯]
• Francesco Pomarico (1960-; Italy) - oboist & conductor [♯]
• Jeffrey Rathbun (Cleveland, OH, USA) - oboist and composer [♯]
• Christopher Redgate (1956-; UK) - oboist and composer, also plays lupophone [♯]
• Jim Rodgers (1967-; Moon Township, PA, USA) - bassoonist, plays heckelphone #4973 (made in 1972) [♯]
• Ernest Rombout (1959-; The Netherlands) - oboist [♯]
• Wolfgang Schottstädt (1953-; Wiesbaden, Germany) - English horn player [♯]
• Werner Schulze (1952-; Vienna, Austria) - bassoonist and composer; used to play heckelphone #3917 (made in 1925)
• Harry Searing (Bloomfield, New Jersey, USA) - bassoonist; plays heckelphone since 2002 [♯]
• Jim Schaeffer (New York City, NY, USA) - bassoonist, artistic director and adjunct professor of business, used to play heckelphone #5021 (made in 2010)
• Emil Sehnert (1871-1940; Dresden, Germany) - bassoonist; one of the first players and early proponents of the heckelphone; played heckelphone #10 at the premiere of Salome by Richard Strauss, on 9 December 1905 in Dresden (Germany)
• Maxine Shimer (1914-1995, Canada/USA) - bassoonist, used to play heckelphone #4107
• William Short (New York City, NY, USA) - bassoonist [♯]
• Andrew Shreeves (New York City, NY, USA) - oboist and musicologist [♯]
• Leland Smith (1925-2013; USA) - bassoonist and composer; student of Darius Milhaud and one of the founders of the Stanford Center for Computer Research and Musical Acoustics (CCRMA)
• Katrin Stüble (Stuttgart, Germany) - oboist [♯]
• Robert Tootelian (1922-2012; USA) - bassoonist, used to play heckelphone #4055 (made in 1928)
• John Upton (St Petersburg, FL, USA) - oboist; has (at least on occasion) played heckelphone #25 (made in 1906) [♯]
• Steve Vacchi (1968-; Eugene, OR, USA) - bassoonist; plays heckelphone #4141 (made in 1930) [♯]
• Tatjana Winkler (~1967-; Germany) - oboist [♯]

That's 56 heckelphonists overall (31 oboists, 1 English horn player, 23 bassonists and one - versatile and famous - clarinettist), of whom those actively playing (as of February 2021) are marked [♯]. Of course, those are just the ones I know about, and I am certain there are (and surely were) many more. OK, perhaps not that many, but at least a few. As I learn about them, I will add them to this list. Contact me at heckelphone@gmx.net or @heckelphon on Twitter to suggest changes or additional entries.