This clinical trial is being conducted at multiple sites to evaluate the activity, safety, and tolerability of XL999 when given weekly to patients with ovarian cancer that has previously been treated with platinum-based chemotherapy. 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.