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 clear cell renal cell carcinoma (RCC). 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.