Hypothyroidism. Pregnancy. Cushing's Syndrome. Prader-Willi Syndrome. Tumor. Polycystic Ovarian Syndrome.
Edema from heart failure, liver failure (i.e. cirrhosis), kidney disease, kidney damage, lymphatic system abnormality, chronic venous insufficiency and other conditions can cause weight gain (due to water retention).
Lots and lots of medications can cause weight gain (i.e. Prednisone, Elavil, Tofranil, Zyprexa, Depot Provera, to name a few).
There are also a lot of psychiatric disorders that can lead to weight gain, but the mechanism of weight gain has to do with caloric intake and activity level.
I'm sure this isn't an all-inclusive list, but I hope it helps!