Wolfram|Alpha has astrophysics calculators. Solve problems related to Jeans length or mass; black hole entropy, radiated power, lifetime, luminosity, surface gravity or temperature; Einstein rings; black hole event horizon radius or area; Kerr black holes; Reissner–Nordström black holes; the Roche limit for fluid or rigid satellites; star energy flux density; stellar burned hydrogen mass; and stellar hydrogen burning rate.
