According to Europol’s 2010 data (PDF) attacks by separatist/nationalist groups far outnumber attacks by Islamists.
I think this is important since the background condition of fairly widespread terrorism on the part of, say, Basque or Corsican terrorists helps us understand the correct context for a lot of Islamist violence, namely nationalism. If you look at Hamas, or the Taliban, or Pakistan-backed radicals in and around Kashmir it should be clear that ethnic nationalism is a major factor in all of these conflicts. It’s true that because Islam is also the dominant religion in the areas in question that the groups have an Islamist ideological orientation that we should construe as sincere. But you also see violence associated with secular nationalism in Europe, with the Tamil Tigers in Sri Lanka. The long (and mercifully waning) conflict in Ireland has tended to break down along sectarian lines, but is normally (and correctly) seen as primarily national in character and not “about” theological disputes between Catholics, Anglicans, and Calvinists—i.e., it’s not a conflict that gets resolved by making people less devoutly religious, and it’s not mediated by compromising over liturgy.