Here’s a good review of recent research on 6 specific programs that made a difference.  The post concludes:

High-quality preschool can be a game changer for all our children, especially for those most at risk of arriving at kindergarten without the skills needed to succeed. The data show that existing large-scale state-run preschool programs are proving effective. The evidence clearly suggests that a federal-state investment that would allow states to grow and improve these existing preschool programs while simultaneously supporting other states as they create their own programs is the wise and prudent path to follow. Given the results highlighted above, encouraging, fostering, and funding preschool programs seems to be the ultimate no-brainer.

via Access to Public Preschool Matters | Center for American Progress.