The Celtic Literature Collective (a website with excerpts of many Irish, Scottish, and Welsh primary sources, both in the original language and in translation)
The Camelot Project, a website on the Arthurian Legend
The Perseus Digital Library, a website with Greek and Roman texts in translation, including those focused on interactions with the Celts and Gauls
The Internet Classics Archive, another website with Greek and Roman texts in translation
The Jacobite Heritage Site (a website with excerpts of documents concerning the Jacobite Revolts in Scotland)
The Irish Potato Famine, primary and secondary sources and images
BBC British prehistory webpage
BBC British history webpage, beginning with the Normans
World History Encyclopedia, a site with articles and maps on many ancient cultures, including the Celts