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 
 A: For Mathematica, you can try Wolfram Alpha:
factor the characteristic polynomial of [[0, 3], [1, 4]]

For Sage, you can try Sagenb.org.  There, you can do
import numpy
n=numpy.array([[0, 3],[1, 4]],'complex64')
m = matrix(n)
m.characteristic_polynomial().factor()

I'm not an expert on this in Sage, but the numpy appears to be necessary to get the polynomial to factor over the complex numbers.
Wolfram Alpha is going to be a lot more user friendly.  Notice I just typed in the description of what I wanted and I got it.  But, it's not as powerful because it's not a full fledged computer algebra system.  If you want to do several steps of calculations where you use a previous step in the next one, it's going to be very difficult or impossible.  But, with Sage, which can be used online for free, or you can download it for free, it will be more complicated to use but more powerful as well, overall.  So, it depends on what your needs are.
A: 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.
A: You also have pieces of Maple online here: 
- via icons: http://www.maplesoft.com/products/StudentApps/index.aspx
- via list: https://maple.cloud/
A: It depends on the size of your matrix. I am not aware of any utilities that are capable of uploading your files and working with them.
If your matrix is relatively small, you can try using Wolfram Alpha. But these days it became quite "heavy" and slower than it used to be, and also has significant limits on computations you can do with it, pushing you to upgrade to the Pro version.
You can use Maxima, Octave or Sage online -- those are free alternatives to Maple, Mathcad and Mathematica. Just Google for it: "maxima|octave|sage online", and you will see services such as maxima-online.org (Maxima), online-utility.org (Octave), sagenb.org (Sage) etc.
maxima-online.org has nice reference with examples. For instance, here is the reference to the function you need: http://maxima-online.org/help/index/charpoly
A: Next to Wolfram Alpha you can also try the Wolfram Cloud on  http://sandbox.open.wolframcloud.com, which allows you to enter and execute multiple Mathematica commands using an online Notebook document.
A: You could use Wolfram Alpha
For example, type in:
characteristic polynomial {{2,3}, {3,1}}
and it will compute the characteristic polynomial for a matrix $\begin{pmatrix}
2&3 \\
3&1 
\end{pmatrix}$
A: I use: http://www.nu-math.com/omega
It is free. There is a quick reference to get you started:
http://www.nu-math.com/nhome/qr.jsp
Here is an example:
m:matrix([2,-1], [3,4]);
charpoly(m, x);
solve(%, x);
