MEK is a critical member of the MAPK pathway involved in growth and survival of cancer cells. PD-325901 is a new drug designed to block this pathway and kill cancer cells. The purpose of this study is to study the effectiveness of PD-325901 in patients with colon cancer, breast cancer, and melanoma. PD-325901 will be given by mouth as a pill twice a day, CT scans will be done and biopsies will be taken of a tumor before and once during treatment to measure the effects of the drug. Blood samples will be taken to measure the amount of drug in the blood.