Downloading ARMA 2 Operation Arrowhead 2010.07.20 from the developer's website was possible when we last checked. We cannot confirm if there is a free download of this software available. This program is an intellectual property of Bohemia Interactive. We recommend checking the downloaded files with any free antivirus. The software relates to Games.
From the developer:
Three years after the conflict in Chernarus, portrayed in the original Arma 2, a new flashpoint in the Green Sea region heats up and coalition forces led by the US Army are sent to Takistan to quickly restore peace and prevent further civilian casualties in this standalone expansion pack to the best military simulator of 2009 – Arma 2.
You may want to check out more software, such as ARM, ARM RVDS or ARM DS-5, which might be similar to ARMA 2 Operation Arrowhead.