Clint Pearson created a flat database project over at CodePlex. Initially I wasn't sold on this but then after a few pokes at the source code, I realized it is actually fairly powerful. Clint implemented a table solution for FlatDB which makes it far more powerful than a typical single serializable class.
Using and saving are dirt simple too!
var db = FlatDatabase("flat.db"); db.Submit();
[ pic via flickr ]