This clinical study is being conducted at multiple sites to determine the activity, safety, and tolerability of XL999 when given weekly to patients with metastatic colorectal cancer (CRC). XL999 is a small molecule inhibitor of multiple kinases including VEGFR, PDGFR, FGFR, FLT-3, and Src, which are involved in tumor cell growth, formation of new blood vessels (angiogenesis), and metastasis.