Mike Frank (48) -Corner OF who appeared briefly for the 1998 Reds brfore Jim Bowden traded him and Denny Neagle to the Yankees for a handful of magic beans.
Gary Green (50) -Light-hitting shortstop who finished his career with the 1992 Reds. Now a minor-league manager. His dad Fred pitched for the 1960 world champion Pirates.
Danny Boone (58) -A descendent of the famous frontiersman of the same name. Washed out of baseball in the mid '80s, spent five years away from the game, then made an improbable comeback by learning a knuckleball and doing well enough in the Senior Professional Baseball League that the Orioles signed him and returned him to the majors at age 36.
Terry Forster (60) -His BaseballReference page lists him at 6'3" and 200, which makes me laugh- he was around 270 in his heyday. Letterman made a running gag out of referring to Forster as a "big tub of goo"; Terry took it good-naturedly. One other thing: Forster could really hit; career 31 for 78, a .397 average.
Sonny Siebert (75) -Indian righty who was drafted by the NBA's St. Louis Hawks but chose baseball. The last American League pitcher to hit two home runs in a game.
Smead Jolley -A big, good-natured lug; a great hitter with a good arm who never became a big league star because managers could never find a defensive spot for him. He was a big star in the Pacific Coast League, though; he hit over .400 in both 1927 and 1928, and in '28 he won the triple crown for good measure. His misadventures in the outfield were legendary; one observer said he chased a fly ball "like a toddler chasing a soap bubble". The famous story about him is that he supposedly made three errors on one play; letting a ball go between his legs, letting it go through his legs again as it caromed off the fence, and then picking it up and heaving it into the stands. This story appears not to be true, but it gives you a sense of his reputation. In the major leagues, he came to the plate about 1800 times and hit .305.