I have so much knit fabrics in my stash (don't ask...), I am really going crazy with making shirts for the girls.
This is one of my favourite FREE patterns at the moment (besides the raglan shirt from "mamahoch2"):
Schnabelina Regenbogenbody. It's originally a bodysuit, but obviously you can also just use it as a t-shirt pattern. Again, the sizing is really accurate, I appreciate that a lot.
There are plenty of sizes available 46 (small newborn) to 146 (teenager). And 3 different necklines. I personally love the "American neckline". In fact I would love a shirt like that myself...
Schnabelina is really creative, there are lots of tutorials and free patterns on her page!
all her patterns: http://schnabelina.blogspot.de/p/tutorials.html