TI-89 series
TI-89 (or TI89) is a powerful graphing calculator developed by Texas Instruments. It has a graphics LCD display with a resolution of 160x100 pixels. It has 256KB RAM (190KB available to the user) and 2MB FlashROM (700KB available to the user). RAM and FlashROM is used to store variables, programs, tables or even games. The heart of the TI-89 is the processor MC68000, which runs at 12MHz.
The TI-89 is fundamentally a TI-92 with limited keyboard, and smaller screen. The TI-89 was created partially in response to the fact that while calculators are allowed on many standardized tests, the TI-92 was considered a computer due to its keyboard. Additionally, many disliked the TI-92 for its large size. The TI-89 is significanytly smaller.
The biggest advantage of the TI-89 is symbolic manipulation, which means that the calculator can compute not only with numbers, but even with symbols. For example if you enter "a*(2+c)" it returns "2*a+a*c" or if you enter "∫(x+1)dx" you get "(x^2)/2 + x". In addition to the normal two-dimenaional plots of functions, it can also produce three-dimensional graphs and implicit plots.
The TI-89 is directly programmable in BASIC. Using a PC, you can also develop your own programs in Assembler and C, translate them to machine language, and upload them to the calculator. Since 1998 thousands of programs for math, electronic, biology, or just for fun have been developed. Some programs are really amazing, like for example a ZX Spectrum emulator or a Chess playing program. Probably the biggest collection of programs and many other stuff is at http://www.ticalc.org/