I've compiled the following list of network anniversary specials, dating back to NBC's "The First Fifty Years" (broadcast on Sunday, November 21st, 1976). I believe it includes every major network special. If I've left any out, please hit the comments with details.
ABC:
"ABC's Silver Anniversary Celebration" (Sunday, February 5th, 1978)
"ABC's 40th Anniversary Special" (Saturday, May 21st, 1994)
"ABC's 50th Anniversary Celebration" (Monday, May 19th, 2003)
CBS:
"CBS: On The Air – A Celebration of 50 Years: The Night of 100 Stars" (Sunday, March 26th, 1978)
"CBS: On The Air – A Celebration of 50 Years: Have a Laugh on Us" (Monday, March 27th, 1978)
"CBS: On The Air – A Celebration of 50 Years: Chills and Thrills with a Laugh Chaser" (Tuesday, March 28th, 1978)
"CBS: On The Air – A Celebration of 50 Years: Dreams Come True" (Wednesday, March 29th, 1978)
"CBS: On The Air – A Celebration of 50 Years: Join the Family" (Thursday, March 30th, 1978)
"CBS: On The Air – A Celebration of 50 Years: We're Getting Personal" (Friday, March 31st, 1978)
"CBS: On The Air – A Celebration of 50 Years: A Rootin', Tootin', Hootin' & Hollerin' Salute to Cowboys and Clowns" (Saturday, April 1st, 1978)
"CBS at 75" (Sunday, November 2nd, 2003)
NBC:
"The First Fifty Years" (Sunday, November 21st, 1976)
"The First Fifty Years-A Closer Look" (Sunday, October 23rd, 1977)
"The First Fifty Years-A Closer Look (Part II)" (Tuesday, January 31st, 1978)
"NBC 60th Anniversary Celebration" (Monday, May 12th, 1986)
"NBC 75th Anniversary Special" (Sunday, May 5th, 2002)
FOX:
"FOX's 25th Anniversary Special" (Sunday, April 22nd, 2012)
I cannot recall any anniversary specials airing on DuMont, The WB, UPN nor PAX/i/ION, and I don't remember the Big 3 airing any specials of this kind prior to the '70s.
I compiled Nielsen ratings for these specials, which have tended to do very well, up until recently:
ABC:
“ABC’s Silver Anniversary Celebration” (Sunday, February 5th, 1978) 27.2HH/40%
“ABC’s 40th Anniversary Special” (Saturday, May 21st, 1994) 7.1HH/12%
“ABC’s 50th Anniversary Celebration” (Monday, May 19th, 2003) 7.6HH/13%, 3.7 A18-49, 11.3M P2+
CBS:
“CBS: On The Air – A Celebration of 50 Years: The Night of 100 Stars” (Sunday, March 26th, 1978) 22.5HH/37%
“CBS: On The Air – A Celebration of 50 Years: Have a Laugh on Us” (Monday, March 27th, 1978) 24.5HH/38%
“CBS: On The Air – A Celebration of 50 Years: Chills and Thrills with a Laugh Chaser” (Tuesday, March 28th, 1978) 24.1HH/43%
“CBS: On The Air – A Celebration of 50 Years: Dreams Come True” (Wednesday, March 29th, 1978) 22.8HH/40%
“CBS: On The Air – A Celebration of 50 Years: Join the Family” (Thursday, March 30th, 1978) 25.2HH/46%
“CBS: On The Air – A Celebration of 50 Years: We’re Getting Personal” (Friday, March 31st, 1978) 19.8HH/35%
“CBS: On The Air – A Celebration of 50 Years: A Rootin’, Tootin’, Hootin’ & Hollerin’ Salute to Cowboys and Clowns” (Saturday, April 1st, 1978) 16.7HH/30%
“CBS at 75″ (Sunday, November 2nd, 2003) 11.2HH/17%, 3.9 A18-49
NBC:
“The First Fifty Years” (Sunday, November 21st, 1976) 28.9HH/44%
“The First Fifty Years-A Closer Look” (Sunday, October 23rd, 1977) 21.5HH/34%
“The First Fifty Years-A Closer Look (Part II)” (Tuesday, January 31st, 1978) 13.5HH/22%
“NBC 60th Anniversary Celebration” (Monday, May 12th, 1986) 18.2HH/28%
“NBC 75th Anniversary Special” (Sunday, May 5th, 2002) 12.7HH/21%, 7.4 A18-49
FOX:
“FOX’s 25th Anniversary Special” (Sunday, April 22nd, 2012) 2.5HH/4%, 1.8 A18-49, 4.3M P2+