Here's something I didn't know: Member states of the European Union are not permitted to place a stamp in the passport of a person who is not subject to immigration control, such as a national of that country, a national of another EU member state or a non-EU national family member of an EU national who is seeking entry in conformity with EU Directive 2004/38/EC. Stamping is prohibited because a passport stamp is imposition of a control that the person is not subject to. This concept is not applicable in countries outside the EU, where a stamp in a passport may simply acknowledge the entry or exit of a person.
I always believed it was a case that countries saw no purpose in stamping passports, it's actually that the EU states that they aren't permitted. Makes perfect sense of course as there is no immigration process for EU citizens. Cool.