MegaCalc has been just customized for the Treo 600. I have it installed and it works great. I believe it is the best replacement for the built-in Treo 600 Calc application. MegaCalc even keeps a log of all of your recent calculations and shows all of the details for each. The graphics alone are a huge step up from the included Calc application. From MegaSoft's website:

- 35 basic math and trigonometric functions
- Mortgage rate calculator
- Tip calculator
- Base and currency converter
- Extra currencies added
- 3 different ways of data input (simple, algebraic, RPN)
- RPN stack viewing
- 4 different modes to display the end result
- Accurate calculation - features up to 18 digits after the decimal point
- Accurate result display - features up to 12 digits after the decimal point
- Calculation range (1e-300, 1e300)
- 10 independent storage cells
- 15 most common constants
- Extra constants adding capability
- Fully supported clipboard
- Graffiti and Palm portable keyboard support
- First-letter search through all of the lists.

It's $14.95 and the newest update means that you don't have to touch the stylus for any of the calculations.