Casio Graphing Calculator (FX-9750GII-WE)If you are on a tight budget, check out the link above for the 9750 GII by Casio. This is the best bargain of all graphics calculators. About the only things I find missing on this 50.00 calculator are geometry and spreadsheet apps. Mathematically, this is a powerhouse.
But, if you have a few extra dollars, you should definitely consider the Prizm. It has all the capabilities of the 9750, plus a spreadsheet and the best portable geometry software I have ever used. The hi-res color screen makes this possible, and it is a real breakthrough.
When you are using your computer (Linux, Windows, or Mac), be sure to download the free software "GeoGebra". The Prizm is a portable machine which does the mathematics of GeoGebra.
And finally, compare the Prizm to the remarkable apps being developed for the IPhone and IPad. The Prizm excels since it has a genuine keyboard and gives such beautiful results. (It also has a battery which lasts 140 hours as compared to the two hours on an IPhone.)
This calculator is truly superior. I use it extensively for solving difference equations,
carrying out statistical analyses, solving calculus problems, and exploring geometrical relationships.
This is the finest graphics calculator I have ever used in my many years of teaching mathematics. It is a beautiful tool to make mathematics really enjoyable.
Update: Jan16, 2011
You might get a Kindle Fire or IPod Touch and use WolframAlpha for CAS and three dimensional graphing. Store your math references on it. Store instruction manuals. But use the Prizm for the daily chores in math.