Hidden Expedition Amazon was already mentioned using the code amazon99. This worked for me, but I also found two more.
I got Treasure Seekers: Visions of Gold for 99 cents using treasure99 and Hidden Mysteries Buckingham Palace for 99 cents using palace99. I would guess these are also good through the end of the month, but am not sure about that. Below are links to these games:
http://www.bigfishgames.com/download-games/4368/treasure-seekers-visions-of-gold/index.html
http://www.bigfishgames.com/download-games/4311/hidden-mysteries-buckingham-palace/index.html