General Information
TI's graphing calculators are commonly used if not required in most high school and college math classes. There are a wide range of models with small feature and price differences.
The entry-level TI-73 is kept simple while still allowing many capabilities of the higher models; it is ideal for middle school and early high school use. The 83+ has the best range of all the models; it is ideal for high school and is very useful through college. The 86, while slightly older, has some advanced capablilites that the 83+ does not, a more avid calculator user may want this model instead. The 89 and 92+ are almost functionally identical; they offer a more graphical interface and are much more advanced, allowing them to be useful well beyond college (in engineering careers), but because of their symbolic solving capabilities, they may not be allowed in many high school and math courses. Other models are still sold but they are older and have less value for their price.
TI's own web site is the best source for getting general information on calculator features. Other calculator fan sites abound the internet as well, and they may also offer views on particular calculators.
TI's graphing calculator site
TI's feature comparison chart
TICalc.org, a central calculator resource
TI-files, another extensive calc site
Dimension-TI, with online purchasing