The Testing by Joelle Charbonneau looks to be free in the US Kindle store right now.
Some other current YA deals, mostly HarperCollins - links to Amazon but I believe they are currently discounted for US readers in other main ebook shops as well.
Unearthly by Cynthia Hand ($1.99 until 6 January)
Under the Never Sky by Veronica Rossi ($2.99 until 6 January)
Angus, Thongs and Full-Frontal Snogging by Louise Rennison ($1.99 until 6 January)
Mind Games by Kiersten White ($1.99 until 9 December)
And then there's
Tomorrow, When the War Began by John Marsden currently at $1.99
Crewel by Gennifer Albin at $2.99
In children/middle-grade,
Warriors 1: Into the Wild by Erin Hunter is $1.99, and
The School for Good and Evil by Soman Chainani is $2.99.