Can One use mathematica or maple online? Is it possible to use some of these algebra packages online ?
I have some matrices that I would like to know the characteristic polynomial of.
Where could I send them to get a nicely factorised answer ?
My PC is very slow & it would be nice to use someone elses super powerful computer !
Any suggestions 

I'll add this as an answer since I can't see how to make it render well in a comment.
You can do this in Sage as Graphth points out, but you do not need to import numpy. You can instead write
F.<x> = PolynomialRing(CC)
M = Matrix([[0,-1],[1,1]])
F(M.charpoly()).factor()

If you want your characteristic polynomial to factor over another field, you can simply replace CC with that field. The real numbers are RR, and the finite field Z/pZ is Integers(p). If you want to factor over the rationals, it's even easier,
M = Matrix([[0,-1],[1,1]])
M.charpoly().factor()

should give you what you want.
