Here's a few to look into:
- Pokémon Diamond/Pearl Damage Calculator by AMF - This online calculator shows the damage at every random number and includes pretty much everything, but it doesn't show percentages. It's simple and very easy to use for quick calculations if you remember/calculate the stats with something else.
- http://www.phyll1s.org/javascript/dp...calculator.htm - I couldn't tell you who made this online calculator, but it's similar to AMF's calculator and shows ranges and percentages of damage. I'll tell you right now: it doesn't work properly in my Internet Explorer 7.0, but it worked fine in Firefox.
- Pokemon Resource by yours truly - I know it sounds like I'm tooting my own horn, but the damage calculator in my resource shows the damage at every random number and shows percentages. It has no Double Battle functionality, though. It's idiot-proof, for the most part. It's also more complex than the other two and (IMO) MetalKid's. It was also made in Microsoft Excel; you'll have to download it. If you don't have Excel, well, it'll still open in OpenOffice, but some work has to be done to make it work properly. I should have an OpenOffice version available come next update.
Crap, just looking at the length of each description, that seems biased, but oh well. Each has their strengths and weaknesses. (I did make more of an effort to show the weaknesses of my calculator, though.)
EDIT: Oh, check Contributions and Corrections for other D/P Damage Calculators. Xenon has one as well as timw06 and Obi, among others.