Pacific Assault will replace last month's free game, 2012's Need for Speed Most Wanted. Curiously, the shooter isn't available free yet; the Origin website says it's "coming soon," and doesn't provide any further details on when the game will actually go free. Pacific Assault regularly costs $9.99 on Origin. Note that Most Wanted is no longer available free, so at the moment, there is no On the House game at all.
On the House is an initiative in which EA offers a full game for free on Origin, its digital distribution platform for PC. Once you've downloaded the game, you have it forever.
Pacific Assault is the seventh Medal of Honor game, and was originally released on Windows PC in November 2004. It was the second game in the franchise to explore the Pacific theater of World War II, following 2003's Medal of Honor: Rising Sun. Both were developed by the now-defunct EA Los Angeles.