You can give Overwatch's latest hero: Sombra, a test-drive on the game's PTR right now.
While Blizzard still hasn't let us know when Sombra will be officially released, you can give her a spin right now in the game's public test realm. To participate, simply select "PTR: Overwatch" from the drop-down REGION / ACCOUNT menu in the battle.net browser.
While Sombra is of course the highlight of this PTR, it also comes with a number of additional features. "The Arcade", which allows players to participate in special rule subsets such 1-on-1 duels, 3-vs-3 skirmishes, or "no hero switching," is also included, as is the Arcade-exclusive new map: Ecopoint: Antarctica.
As stated previously, this iteration of the PTR will have much more significant character balance changes, and will feature much more frequent updates. You can keep up-to-date with all of those tweaks here.
Lastly, a few "quality-of-life" changes are being tested out on the PTR that everyone should rejoice in. First up, the amount of experience needed to reach levels 14 through 100 has been slightly reduced, the amount of experience needed to reach levels after 100 is now constant (20,000 XP) and will no longer reset after each promotion, and last and most certainly not least, Quick Play games will also be restricted to one-hero per team, just like in competitive.
You can check out the full PTR patch notes here.