My go-to for these things is wolframalpha.com, mainly for ease of access, and the natural language interface means I don't have to remember any syntax. You can input queries like "complete the square ax^2 + bx + c" or "derivative of exp(x^3)" and get solutions, it's a real timesaver. Also, for vector/matrix derivatives, http://www.matrixcalculus.org/ is amazing.