The Kosmos two-player series of games (the "Spiele für Zwei" in German) contains many popular, quality two player games that are, for the most part, easy to learn and play. The games in this series often show up in "What to Play With Your Spouse" GeekLists on BGG. Their small box also lends itself to portability. Many of the series have been published by Rio Grande Games except where noted.
Rosenkönig (no English edition - German version with English rules in box sold by Mayfair Games) (new edition released in 2007, with possible English edition in the future)
Sakkara (English edition was announced by Rio Grande Games, but Jay Tummelson later expressed concerns with the design and postponed release pending corrections from the designer. Current status unknown.)