Thanks bmer. I think I'm going to head most of the vast replies off at the pass and answer you. They didn't make as many contributions to our intellectual history because they didn't keep large libraries. Many of their languages didn't have a written form, and though their leaders (I'm speaking of European tribes here) could usually read and write, it was usually only in Latin or Greek.

They DID conquer and maintain sizable territories, the best of which were taken (or disrupted) by the Romans. Most people don't know about these confederations or kingdoms because they get cut from history courses required for compulsory education (K-12) to focus on things educators and governments think will make people better citizens. Some areas stayed disintegrated, where individual tribes held prominence, much like European Greece. A few barbarian tribes had ugly customs such as human sacrifice, cannibalism, etc., either for the same reasons other people have done such things in history (the ritual power of such these atrocities: taking your enemy's strength through their flesh, the importance of blood and using it to appease gods, much like the Aztecs), or because Romans and Greek writers made up the fact that barbarians had these customs because they were relying on rumors of rumors or trying to make them look scary people who should be killed to justify a war.

EB only mentions cannibalism in reference to the Vojinos, where there is at least some archaeological evidence to back up assertions, and reminds us that the Germans and Celts were using soap more often than their neighbors at this time.

Also, the Romans were not above the occasional human sacrifice at rare moments in their history...