Play Diablo 3 Beta for free

After downloading and installing the Diablo 3 beta you can play it for firee from Friday, April 20 at 12:01 p.m. PDT (noon), until Monday, April 23 at 10:00 a.m. PDT .

What steps do I need to take?

1.     Log in or create a Battle.net account at http://www.battle.net (you can use even a free acc.)
2.     Create your BattleTag™, which is required to play Diablo II
3.     Download the beta client, install, and play between 12:01 p.m. PDT April 20 and 10:00 a.m. PDT April 23

You can read the full blog-post from blizzard here.