I saw this this other day under new releases. Shacknews mentioned that Heroes of Newerth is free to download until May 12, which is Wednesday, or tomorrow. After that it will be released as a retail version and you’ll have to pay for it.
It was a NAG Online forum post which reminded me of all this though.
This seems to be the beta version. The game is available for Windows, Mac, and Linux. You can download the game, which weighs in at 340 MB, at the following places. You’ll have to create an account in addition to downloading the title.
Source: Heroes of Newerth going retail 12 May 2010 last chance to play for free [NAG Online Forums]