From now (well, June 12th, but this news got lost among the E3 hubbub), through July 9th, you can earn all sorts of rewards by tuning into Heroes of the Storm streams on Twitch. All you need to do is link your Blizzard account to your Twitch account.
The big reward (the one running from June 12th to the 18th, unfortunately) is from tuning into the Midseason Brawl for at least an hour each day. From there, 2 winners per day will be given Heroic Accounts, unlocking every hero currently available in game (including Yrel).
Everyone can earn 1 rare loot chest by watching 3 hours or more of any Twitch stream between now and July 9th. 6 Hours will get you a portrait.
Finally, between now and July 14th, watch HotS streams to randomly win loot chests – Blizzard will be giving out 1000s.