Recommend a game... win a prize 2 :D

The only Beta I'm part of right now is Starfarer and I gotta say it is one great game in project. Updates are rather slow to come, but always big.

http://www.youtube.com/watch?v=Wc0fWh0_Y28 Check this it looks really awesome, preview made by TotalHalibut. Game name City of Steam and it's alpha started just two days ago.

Also other games mentioned are good, like Dota2 or Counter Strike Global Offensive, Path of Exile and others.

Dota 2. It is really good. It may seem hard but in no time, you will get a hang of it. Trust me. You will never know what happens in the game until it ends.

Moonbase Alpha...

Here hope you enjoy it its a nice next game

There are two really good looking alphas:
Overgrowth, A fun new game from wolfire games, you play as a human sized rabbit and go and sneak attack and fight other rabbits.

And

Rambros, A super fast paced awesome pro america shooter, with hats and rockin' music and crazy battles.

Powder toy.
you liked gnomoria that is a sandbox theme,powder toy is neither alfa or beta,but i assure you,if you learn to play,you can have a lot of fun making strange things.
https://www.youtube.com/watch?v=lzoKcMD2x58&feature=related
https://www.youtube.com/watch?v=KKCtvAChaRE&feature=related
here's a pair of video of guys destroying a city

skyrim,css, and garrysmod

http://www.townsgame.com/ is quite fun :)

http://www.youtube.com/watch?v=YGy8yWXlOjQ more, other game, beta coverage from TotalHalibut, enjoy the video.