I'm not sure if this counts as a "draft gem" since Arian Foster went undrafted in the 2009 NFL Draft, but we're going to count it anyway because he was a gem for the Houston Texans for seven seasons. He was also a pivotal part in the Texans winning four AFC South titles.
Foster earned four Pro Bowl selections and one First-Team All-Pro selection in 2010 when he led the league in rushing yards with 1,616 rushing yards and 16 touchdowns. He also led the league in rushing touchdowns in 2012 with 15 touchdowns.
He is also regarded as the best running back in Texans franchise history. While the Texans have only been an NFL franchise since 1999, that's still an impressive feat for an undrafted free agent out of Tennessee.