Cancer prognosis is of keen interest for patients with cancer, their caregivers, and providers. Prognostic tools have been developed to guide patient-physician communication and decision-making. Given the proliferation of prognostic tools, it is timely to review existing online cancer prognostic tools and discuss implications for their use in clinical settings. Using a systematic approach, we searched the Internet, Medline, and consulted with experts to identify existing online prognostic tools. Each was reviewed for content and format. Twenty-two prognostic tools addressing 89 different cancers were identified. Tools primarily focused on prostate (n = 11), colorectal (n = 10), breast (n = 8), and melanoma (n = 6), although at least one tool was identified for most malignancies. The input variables for the tools included cancer characteristics (n = 22), patient characteristics (n = 18), and comorbidities (n = 9). Effect of therapy on prognosis was included in 15 tools. The most common predicted outcome was cancer-specific survival/mortality (n = 17). Only a few tools (n = 4) suggested patients as potential target users. A comprehensive repository of online prognostic tools was created to understand the state-of-the-art in prognostic tool availability and characteristics. Use of these tools may support communication and understanding about cancer prognosis. Dissemination, testing, refinement of existing, and development of new tools under different conditions are needed.