New Keynesian Economics

While early Keynesians were comfortable simply postulating that wages and prices were slow to adjust as a practical reality,

Theoretical response to the rational expectations criticism.

Sticky Price (Menu Cost) Models

Efficiency Wage Models

Insider-Outsider Models and Hysteresis

Imperfect Competition

Staggered Price Adjustment

References

New Keynesian Economics, Gregory Mankiw, Concise Encyclopedia of Economics.
New Keynesian Economics, Wikipedia.