1. July 25, 2001: A's receive outfielder Jermaine Dye from the Royals in a three-team trade. The Rockies sent infielder Neifi Perez to the Royals and Oakland sent Minor League outfielder Mario Encarnacion, Minor League infielder Jose Ortiz and Minor League pitcher Todd Belitz to Colorado.
The A's got exactly what they needed out of Dye at the time of the trade. In 61 games, he hit .297/.366/.547 with 13 home runs and 59 RBIs, helping the team to a 102-60 record, which despite being 14 fewer victories than the division-winning Mariners, was good enough for the American League Wild Card spot.
Dye was a bit inconsistent in his final years with the A's, but Oakland still came out ahead in that deal.
2. July 23, 1999: A's receive outfielder Terrence Long and Minor League pitcher Leonar Vasquez from the Mets for right-hander Kenny Rogers.
Long, the Mets' 1994 first-round pick, made his debut with the A's in 2000, setting an Oakland rookie record with 104 runs and tying the rookie record with 168 hits en route to finishing second in AL Rookie of the Year Award voting.
In 2001, Long and Miguel Tejada started all 162 games, helping Oakland to a playoff appearance.
3. July 31, 1999: A's receive right-hander Jason Isringhausen and right-hander Greg McMichael from the Mets for right-hander Billy Taylor.
Isringhausen established himself as a top closer when he came to Oakland, earning his first All-Star selection in 2000 and helping the club to the playoffs that year and the next.
He compiled 75 saves during his two-plus-year tenure with the A's.
4. July 8, 2008: A's receive infielder Josh Donaldson, left-hander Sean Gallagher, infielder Eric Patterson and outfielder Matt Murton from the Cubs in exchange for right-hander Rich Harden and right-hander Chad Gaudin.
For years this deal favored the Cubs, but Donaldson's breakout performance with the A's this season may change that, especially given that Harden came back to the A's in 2011.
5. July 29, 1999: A's receive infielder Randy Velarde and pitcher Omar Olivares from the Angels in exchange for Minor League outfielder Jeff DaVanon, Minor League outfielder Nathan Haynes and Minor League pitcher Elvin Nina.
Velarde finished fourth in the AL in hits (200) that year, while also posting career bests in average (.317), runs (105), home runs (16), RBIs (76), stolen bases (24) and on-base percentage (.390). Then, in 2000, he helped the A's to a division title.