Look down at Georgia Tech. Jason Neighborgal has hit that velocity repeatedly. Bob Feller is probably regarded as the fastest ever that made it to the majors. He personally claimed he hit 104 MPH.
Nolan Ryan was "officially clocked" back in the 70's by the Guinness Book of World Records at 100.9 MPH.
Bob Feller once traveled to the Aberdeen proving grounds to get clock by the Military (This is prior to the advent of "Radar Guns"). They clocked him at and 98.6 MPH, throwing without a mound.
Ted Williams claimed that Steve Dalkowski was the hardest thrower that ever lived. He stated that he was faster than both Walter Johnson and Bob Feller. He was estimated to have reached 105 MPH, even though he was only 5'8". In the minors, he walked 1350 baters while striking out about 1400. He walked 21 batters in one minor league game and struck out 21 in another. He once threw 120 pitches in only two innings.
One of the early forms of determining pitch speed was by measuring the speed against a motorcycle flying past. That's how they clocked Walter Johnson back in the early 1900's at 99.7 MPH.
After the advent of the Radar gun, the fun was taken out of it.
Her are a few of the perhaps 50+ members of the highly unofficial "100 MPH club" in Major League Baseball.
Mark Wohlers, 103.0 mph, 1995
Armando Benitez, 102.0 mph, 2002
Randy Johnson, 102.0 mph, 07-09-2004
Robb Nen, 102.0 mph, 10-23-1997
Rob Dibble, 101.0 mph, 1992
Kyle Farnsworth, 101.0 mph, 05-27-2004
Eric Gagne, 101.0 mph, 04-16-2004
Jose Mesa, 101.0 mph, 1993
Guillermo Mota, 101.0 mph, 07-24-2002
Billy Wagner, 101.0 mph, 04-16-2004
Billy Wagner, 101.0 mph, 06-11-2003
Nolan Ryan, 100.9 mph, 08-20-1974
Josh Beckett, 100.0 mph, 10-12-2003
Roger Clemens, 100.0 mph, 10-10-2001
Francisco Cordero, 100.0 mph, 07-07-2004
Jorge Julio, 100.0 mph, 09-16-2004
Ben Sheets, 100.0 mph, 07-10-2004
J.R. Richard, 100.0 mph, 1976
C.C. Sabathia, 100.0 mph, 2002
There are lots of sites you can go scan to research it. One of the best sites to find out this kind of information (and one of my favorites) is the Baseball Almanac If you've never visited it before, you can loose yourself in there for a couple days.