4. Jake Plummer, QB (2003)
Jake Plummer doesn't get nearly enough love as one of the best free agent pickups in Denver Broncos history. Although he was only with the team from 2003-06, Plummer undoubtedly made his mark.
When he was signed in the 2003 offseason, the Broncos were a few years removed from winning a Super Bowl in the 1998 season. The team had tried to move on to its own draft pick -- Brian Griese -- after John Elway retired, but it didn't work out. The 2002 Broncos were 9-7 as a team but in four years as the starter, Brian Griese had thrown 52 interceptions including 34 from 2001-02.
The Broncos decided to sign Jake "The Snake" when he became available after starting his NFL career with the Arizona Cardinals, and the 2003-06 Denver Broncos included some of the most complete and well-rounded teams in Broncos franchise history.
Specifically, the 2005 Broncos had "The Snake" in some MVP talks for a bit. Plummer made the Pro Bowl in 2005 as the Broncos rolled to a 13-3 regular season record. He set a franchise record that year for passes thrown without an interception. Although that team got to 13-3 much differently than the 2013 Denver Broncos, that offense was an absolute blast to watch every single week, seemingly having their way with any defense and Plummer operating the whole thing in-step with Mike Shanahan and Gary Kubiak masterfully.
That team would become the first to ever beat Tom Brady in the playoffs. Unfortunately, that was Plummer's only win in the playoffs as a Bronco. Despite a stroke of good fortune that landed the Broncos a home game in the AFC Championship against the Steelers, they collapsed and the Steelers went on to win the Super Bowl.
Plummer's time as a Bronco ended rather unceremoniously when he was benched for Jay Cutler, but the Broncos were 39-15 with Plummer under center, and he brought the franchise to its highest point post-John Elway (at the time).