If you're neck-deep in math work, or just nostalgic for the high school days of your TI-83+, fire up the big G: it'll now serve you as a graphing calculator. Time to make sine waves!
Who needs Texas Instruments? Not Matt Stack, creator of the Open SciCal. His homemade, 100% open source graphing calculator not only blows away the functionality of store-bought devices, but is the "ultimate status symbol among the nerdiest of the nerds."