I am a citizen of an EU member state, being a resident in another EU member state. I hold a both a national identity card of my birth country and a resident permit of my home country. In an answer to an earlier question I asked about traveling to the UK with an almost invalid national identity card it was mentioned that at least in the UK I should be given "every reasonable opportunity” to prove I am an EEA national. Given that information is printed on my resident permit, I am wondering to what extend such a resident permit is a valid travel document to travel within the EU.
You can enter any EU/EEA member country as long as you can prove your nationality by any means, according to Wikipedia:
Strictly speaking, it is not necessary for an EEA or Swiss citizen to possess a valid national identity card or passport to enjoy the right of free movement. In theory, if an EEA or Swiss citizen can prove his/her nationality by any other means (e.g. by presenting an expired national identity card or passport, or a citizenship certificate) he/she must be permitted to enter and reside in the EEA and Switzerland without a visa.
As you can see, as long as you can prove your nationaly by any means then you can travel to any EU/EEA country. Of course presenting a valid passport or national ID card should make things easier but any other document would work as well.