I'm aware that Ogham was used in Ireland, and Runes in certain Germanic areas, and Greek was used and still is used today.
I'm interested in how the people of Britain, Gaul, Iberia, the Baltics, and Slavic areas wrote before the Roman Empire and later kingdoms popularized the Latin and Cyrillic scripts?
I only know for Slavic so I will answer only for them.
They were completely illiterate (short answer).
Long answer:
Serbia, Bulgaria, Macedonia, Russia, Belarus, and Ukraine, the role of Cyrill and Methodius is overstated (most likely) to say that they were the first who actually developed Slavic literacy. Thing is, the time frame matches (9th century) and they did develop Cyrillic, so it is possible as it coincides with the birth of the earliest written Slavic language (Old Church Slavonic).
Reason for sceptisism: first Slavic recorded language was in Great Moravia (modern day Slovakia), quite a distance from Bulgaria, and no recorded proof of these documents remains, only what the contemporary Byzantine Emperor said at the time. To guess what that was written in, is science fiction. Again though, time frame is right there (9th century).
If this is true, then Slavs wrote with Cyrillic to start with.
According to Caesar in his commentaries, the Druids in Gaul deliberately did not write down their beliefs for they believed it lessened their political power and prestige. Having said that, there have also been coins found in Gaul that have been written with a Latin script. I believe they were minted with Dumnorix's name on them.
Sources: De Bello Gallico, by Julius Caesar Caesar: Life of a Colossus by Adrian Goldsworthy
In Ireland we used Ogham,
http://en.wikipedia.org/wiki/Ogham#Alphabet:_the_Beith-Luis-Nin