The purpose of this study is to determine whether or not XL184 demonstrates anti-tumor activity in selected tumor types under a randomized discontinuation trial (RDT) design. Subjects who have responded to study drug after 12 weeks of open-label XL184 administration will continue to take XL184. Subjects who are clearly progressing will discontinue study treatment and subjects who demonstrate stable disease will be randomized to either XL184 or placebo. For individual patients, once disease progression is observed, the blind will be broken and subjects who were randomized to placebo will be offered the option to receive open-label XL184. Subjects who progressed while taking XL184 will discontinue study treatment.