Hverra manna ert þú?
Iceland: I guess when you live in an isolated country of only 300,000 people who don't use surnames, then the chance of an incestuous relationship must be pretty high, especially for those from smaller communities. This is where Íslendingabók ('The Book of Icelanders') comes in. Íslendingabók is a genealogical website linked to a database containing information on almost every Icelander since the 18th century.
To use the site (which incidentally is free), you simply input your name and that of your potential partner, and the database will return information about any family ties. Sounds like an excellent idea.
For more information about Íslendingabók (in English!) see here.