There are both commercial software and freeware for playing Diplomacy.
Freeware is software that is free to the public to use. Freeware software allows people to use and access the programs free of charge and may be shared with other individuals, if permitted. The right to freeware is often held by the owner, meaning no one is able to alter, change, or sell the created software. A common example of freeware is Preezo (similar to Microsoft PowerPoint), which is a presentation software that is free via the Internet. Like Preezo, freeware software is very commonly shared on the Internet for public use.
phpDiplomacy is free software that lets you play online via your web browser.
jDip is a very commonly used Diplomacy mapping/ajudication application, with many features and variants.