A Farmall "Regular" was the first of the International Harvestor (IH) "rowcrop" tractors. Introduced in 1924 (actually, 1923, but the first were sold only in Texas...), the regular was produced until 1932, when it was replaced by the "Increased Power" Farmall (later called the F-20).
http://www.tractordata.com/td/004/td4685.html
It truely revolutionized farming in the 1920's, and influenced it for decades. The Farmall "Letter Series" (A, B, C, H, M), and "Hundred Series" (#300, 400, etc, etc) are direct decendants of the "Regular".
Interestingly, it was never "officially" named the "Regular". It was introduced as "The Farmall". People started refering to it as "Regular" after the F-20 started production (as the "Increased Power" Farmall), to keep the two separate on paper.
Along with the 9N Ford in 1939, the Regular is considered to be one of the top two tractors of all time in the amount of influence it had on reshaping the farming world (tractor wise..).