You can use gap, kash, maxima, singular, and gp
through the SAGE interface. For example,
print gap('SymmetricGroup(7)')
print kash('NumberField(X^3 + 15*X^2 - 15*X + 15)')
print maxima('tan(x)').integrate('x')
print singular.ring(0, '(x,y,z)', 'dp')
print gp('ellinit([1,2,3,4,5])').j
There's also functionality of mwrank, gfan, polymake, and everything
else standard in SAGE.
NOTE: Use of the Magma, Mathematica, and Maple interfaces
via SAGE this way has been explicitly disabled for legal reasons.
However, online access to Magma is
available at this site.