The Half-Life 2 expansion pack won't be a steam only release.

According to British website Eurogamer, Valve's Doug Lombardi has confirmed that the Half-Life 2 expansion pack, Aftermath, will be released at retail, but a publisher has yet to be confirmed. Vivendi Universal Games published Half-Life 2, but the company will no longer publish Valve software from the end of August.

The game will still be made available to download via steam when it's released sometime in September (that is the release date according to Valve, but we know how these things can change). No price for the expansion has been announced.