Welcome to DotACalc.com - Alpha version/Updated to 6.54b (Working on 6.55)
This online tool lets you calculate DotA Allstars heroes' attributes that are hidden in the game and that can be hard to figure out such us effective hit points due to armor or damage per second factoring attack speed, apart from the basic values and many extras like item incompatibilities and total cost of inventory. You can open many tabs if you want to compare heroes or builds, and the address is always linkable, just use the url on the address bar of your browser at any time to link to the build you are seeing in your guide or blog. To get started just choose a hero (Sentinel are up, Scourge are down), the rest is easy to figure out.
This tool is still in development so it certainly contains many inaccuracies or bugs, if you are aware of any of such errors please drop a message by email ("contact" @ this domain) or in the official thread at the DotA forums, I'll be sure to credit you in the future acknowledgements section if the error that you report gets fixed.
Note: The current state of the tool is alpha/preview, it still doesn't take into account special hero abilities, but that it's in development and it will be available very soon. Also, the description of some skills may be outdated.