July 31, 1998: Randy Johnson, Mariners to Astros
Aside from the Brock trade, this one had the biggest impact of any deal mentioned here. The Big Unit, who had won the 1995 AL Cy Young Award and led the Mariners to their first postseason berth that season and to their first division title two years later, was upset that Seattle hadn't offered him an extension beyond the '98 season. Johnson made it known that he would test free agency, and when both he and the Mariners were roughed up in 1998 (he had a 4.33 ERA in 23 starts, they were 48-59), the team traded him to Houston for righty Freddy Garcia, infielder Carlos Guillen and a player to be named later (lefty John Halama).
Johnson thoroughly dominated the NL, going 10-1 with a 1.28 ERA in 11 starts for the Astros, who pulled away from the NL Central pack for their second of three straight division titles. As with those other years, they were bumped off in the Division Series, this time against the Padres. Johnson lost both of his starts despite allowing just four runs while whiffing 17 in 14 innings. After the season, he would sign a four-year, $53.4 million deal with the Diamondbacks and begin the most dominant stretch of his career: He won the Cy Young in each season and helped Arizona win the World Series in 2001. That same year Garcia, Guillen and Halama helped the Mariners win a record 116 games, though they lost in the ALCS to a Yankees team that would then be felled by Johnson and the D-backs in the Fall Classic. Johnson finished his career with 303 wins and 4,875 strikeouts, second all-time; he was elected to the Hall with 97.3% of the vote in 2015.