Blizzard Entertainment finally revealed the 23rd hero in Overwatch, Sombra, at BlizzCon 2016. Originally rumored to be part of the founding team of Overwatch (the good guys), she is actually part of the shady Talon organization. With her right click, Sombra can disable a target, while her ultimate is an AOE disabling shields (Lucio ultimate, Reinhardt shield etc) and turrets. Oh, and she can stealth too!
Update: Blizzard has issued a takedown notice for all my BlizzCon videos except for big partners like IGN, so here are the videos from IGN.