<p>You can also check you the new TI-nspire series, especially the CAS edition (equivalent to the ti-89). They are much more advance that the previous models (and slightly more expensive), but they are worth it.</p>
<p>You can also look at the HP calculators. I have heard very good things about them and some people absolutely swear by them. I've always used the TI-86 and the TI-89.</p>
<p>I love the Ti-89. It does calculus for you and you can load programs onto it that do differential equations for you. Plus the games are better than Ti-83's.</p>
<p>The most powerful calculators on the market are the Texas Instruments TI-89 and the Hewlett-Packard HP-50G. </p>
<p>The TI-89 is easier to use and is very popular with undergraduates (most of whom are familiar with TI graphing calculators from high school). </p>
<p>The HP-50G is more powerful and customizable than the TI-89; however, it is also more complicated, and more difficult to learn to use effectively. It is really designed for experienced professionals, not students who are learning about calculus or linear algebra for the first time.</p>
<p>The TI-Nspire has some impressive capabilities for classroom use, but is surprisingly poor for programming. It therefore lacks the flexibility of a conventional graphing calculator. </p>
<p>The rumor is that TI crippled the programmability deliberately, to the Nspire from being used as a platform for games.</p>
<p>Don't know if they are made any more, but I have a HP-15C. Bought this in 1981 and have used it almost daily since. It got me through engineering and MBA school as well as numerous jobs. Probably my most reliable possession.</p>
<p>I will second the HP-15c (they don't make them anymore, but go for a couple hundred on eBay). I inherited one, and it's the only "calculator" I use for my classes. If I need to do anything more complicated than scientific computations, I use mathematica. Ultimately, it doesn't matter how powerful the handheld calculator is, you still have to type on it- and a computer keyboard is usually a lot faster than a handheld calculator, especially when you have to deal with variables and functions.</p>