A comprehensive online collection of Celtic historical and literary texts. Be warned - the translations are serviceable but not the best!
Search for any Welsh place name
All of Dafydd ap Gwilym's poems edited and translated
Irish texts published by the Dublin school of Celtic Studies. Includes every version of the Irish Annals
Online dictionary - provides all grammatical possibilities. Available for Old Norse too.
This "web resource for the advancement of knowledge and understanding in areas related to heathen lore and literature" offers a number of primary texts both in the original and in translation, and also some secondary material.
If you read Russian, you might also want to have a look at the main page (or try navigating it via a website translator).