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You can, but don't expect to be able to use it on any test in college, high school, or the SAT.

If you have an Android device, HandyCalc is free-beer and very, very good.

If you need serious calculating, there are all sorts of open source packages for Linux, and a number of devices of varying capabilities to run it on.

I actually think the problem with all of these solutions is standardized testing. Throughout high school, I used my iPhone as a calculator for everything, but whenever we needed one for a test I had to drag out my old TI, because people were afraid I would cheat if I had internet access.

Octave is pretty awesome. There is also R. Personally, I also do a lot of computing within interactive programming console (like irb, php -a, node). However, if you are doing symbolic expression, you can't go wrong with Octave