+1 -- Maxima is used under the hood for many key symbolic functions in Sage, e.g., limits, symbolic integrals, many simplifications, etc. We Sage devs (mainly Nils Bruin) also wrote a very efficient C-level bridge interface between Python and Maxima, which uses ECL (embedded common lisp). We also try to make Maxima easy to use from Sage, e.g., you can type "maxima(sage expression)" to convert an expression to Maxima. Maxima developers regularly post on the Sage mailing list too.