I was searching for scientific calculator and came across this one: gCalc!
I haven't seen this posted here, so I thought I would! To download it, just go to downloads at the top, and choose gCalc (or their other apps if you so desire) and download the OTA or .zip file. I just did it (I have OS 4.5 on my 8310) and it works like a charm! I absolutely LOVE IT!!!