With a lot of help from aldaron and Ruesap, I've written a damage calculator (in PHP, for anyone who cares) for Pokemon. It can be found here: http://www.ownpurpose.com/damage_calculator.php
My goal is to make this the most accurate damage calculator for pokemon on the internet, so I'd appreciate it if you'd send me any bugs/errors you may encounter, even if it's an oh so small one, via PM or IRC or, and this is probably the most beneficial way, post in this topic (this prevents people from sending me the same error twice or something like that). Either way will work.
What it does at the moment is it asks you for some data (attacking pokemon's (Sp)Atk, defending pokemon's (Sp)Def, base power of the move, etc...). Just fill in these boxes correctly and you'll be alright. Next there are some dropdown menus with things that can affect the damage calculation, like hold items, abilities, stat boosts, you name it. There's also a separate box for the opponent's HP stat and the opponent's weakness to Stealth Rock. These fields are used to calculate the odds of your attack OHKOing, 2HKOing or 3HKOing the opponent with the given stats.
If you really want to get familiar with it, I'd suggest just playing around with it a little bit.
Now, if you do encounter some bugs or errors and you've decided to notify me, please make sure to include the exact data you filled in. Please don't just say "the calc seems to calculate 2HKO odds improperly". It'd be much easier for me to fix any bugs if I know what input you used, so please, include that in your PM or in your message on IRC.
What I'd also like to ask you is to try the exact same calculation that caused an error with the calculator found in the link above here: http://www.ownpurpose.com/damage_calc.php
Why? Because the first calculator in this topic is a newer version, it's been upgraded from the one I just posted, but as such, it's highly possible that I've made things that worked previously generate errors now. If your calculation seems to be correct when you tried it with the second damage calculator, that will narrow down the amount of possible mistakes in the coding and thus it'll be easier for me to fix said mistakes.
Thanks for now, and have fun with this thing.
PS: there's a stat calculator on the site too, but at the moment, I'm more interested into fixing the damage calculator. If you find some bugs in the stat calculator, do notify me anyway, because I can always just archive those bug reports or do something about it right away if I have some spare time (which I won't have plenty of the coming weeks, sadly :().