Opens Sans (excellent) @12px base starting scale
Lora (excellent) @12px
PT Sans (excellent) @12px
Cabin Condensed (very good) @13px
Philosopher (good) @13px
Istok Web (very good) @12px
Arimo (good - excellent for body copy) @12px
PT Serif (good) @13px
Arvo (could work if paired with a good sans. leading issues though) @12px
Gentium Basic (ok) @15px
Droid Serif (ok) @12px
Amaranth (pretty good, needs to be paired) @13px 
Cuprum (ok) @14px
Signika (ok) @13px
PT Serif Caption (Good) @12px
Lato (goodish) @12px
Expletus Sans (good) @12px
Dosis (goodish) @14px
Signika Negative (good) @13px