Here are two notes on factoring efforts circa 1950 and also 1990.
The former is an excellent intro to the latter.
Here is a brief description of D.H. Lehmer’s factoring engines that were running about 1950. I was a student of Lehmer in about 1953. I think that Lehmer factored a hard 40 digit number with the wheels.
Next more recent general purpose computer factoring efforts. I think that these factored a 140 digit number.
A report of complete factorization of 21039 − 1 using special properties in 2007.
Here is a report on factoring a hard 768 bit number in 2010.
Factoring π, a challenge.