Graphing Calculators for Geeks?

Winged Plum asks: "It's time for me to get a graphing calculator for my high school classes, as they're mandatory for next year's courses. After looking at some from TI and others from HP, I've got one question: Which is the best for a geek? The TI is pretty much the education standard, but HP claims to have some great calculator programming stuff. (They call it the Most Programmable Calc out there.) So should I go with the flow and get a TI, or should I get an HP and have a little fun with it? Any suggestions?" Are TI and HP the only ones out there doing graphing calulators? If not, who else in the market makes some decent products?