The usual suspects
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.
Zaid Jilani, also at Think Progress, notes that the same is true in America:
As ThinkProgress noted at the time of his first hearing examining exclusively the radicalization in Muslim communities, there have been almost twice as many terror plots from non-Muslims than Muslims in the United States since 9/11.
Bruce Bawer, who recently returned to the U.S. after living for years with his partner in Oslo, writes in WSJ that, while he at first assumed that Islamic terrorists were behind the Oslo attacks, he was not surprised when it turned out that the culprit was a Norwegian Islamophobe:
Several of us who have written about the rise of Islam in Europe have warned that the failure of mainstream political leaders to responsibly address the attendant challenges would result in the emergence of extremists like Breivik.
But I was stunned to discover on Saturday that Breivik was a reader of my own work, including my book While Europe Slept: How Radical Islam Is Destroying the West from Within. In comments posted in 2009 on a Norwegian blog, document.no, Breivik expressed admiration for my writings, but criticized me for not being a cultural conservative (although he was pleased that I was not a Marxist, either).
Later on Saturday came news of a 1,500-page manifesto, entitled "2083: A European Declaration of Independence," that Breivik had recently written and posted online. The first half, in which he indicts the European cultural elite for permitting Islam to take root in Europe, makes it clear that he is both highly intelligent and very well read in European history and the history of modern ideas.
In the second half he describes himself as having revived the Knights Templar. He also outlines in extreme detail how he and his fellow anti-jihadists can acquire weapons, ammunition and body armor and thereupon proceed to use "terror as a method for waking up the masses" to the danger posed by Islam. This makes it clear he is completely insane.
Bawer's main concern still seems to focus on Muslim extremists in European countries. He laments, "It will, I fear, be a great deal more difficult to broach these issues now that this murderous madman [Breivik] has become the poster boy for the criticism of Islam."
I myself have written about radical Islamists and how gay leftists often refuse to criticize them and call such criticism Islamophobic, disregarding the virulent homophobia of the Islamists themselves. Indeed, Bawer quoted me on pages 193 and 194 of his 2009 book, Surrender: Appeasing Islam, Sacrificing Freedom. But Bawer needs to step back here and put aside his Muslim-centric lens long enough to recognize that nativist, xenophobic and religious-motivated violence by native-born Americans and Europeans is a serious problem in its own right and should not be minimized by treating it mainly as an unfortunate diversion from the proper focus on Islamists. (He might deny intending any such meaning; but he is already horrified at having been misused by Breivik, so perhaps this is a good time for all of us who have ventured into these waters to broaden and reframe our messaging.)
If we really want to end violence rather than merely indulge in online flame wars, we need to seek ways to illuminate these contentious issues and resist hot-button rhetoric that effectively shuts down (or cartoonizes) discussions before they can properly start.
For one thing, we can learn from the differences between America's experience with Muslim immigrants and Europe's. The United States has been much better at assimilating immigrants than Europe has. The problem of Muslims in European cities who refuse to learn the local language and demand to be governed by Sharia law is largely the result of liberal post-colonial policies that ghettoized immigrants and thwarted the assimilation process that would have given them more of a stake in their new countries.
In any case, the evidence shows that both Americans and Europeans have far more to fear from home-grown terrorists driven by racist nationalism than from immigrants. In the U.S., gay people have been largely replaced by Muslims and undocumented "aliens" (to use a prejudicial term) as the right-wing's favorite election-year scapegoats. I believe that those of us who oppose nativism, xenophobia, and religious bigotry need to build coalitions with our fellow scapegoats not in a naive expression of solidarity with people who hate us, but in a clear-eyed recognition of both our differences and the diversity within each population. I (to cite a few small personal examples) have donated to the much-demonized Islamic Center in lower Manhattan, as well as to the re-election campaign of Rep. Keith Ellison (D-MN), co-chair of the Congressional Progressive Caucus, who is both pro-gay and Muslim; and I have written against the demonizing of Muslims. Fostering trust and cooperation between communities takes time; but speaking out and organizing against the partisan exploitation of xenophobia and religious bigotry, while taking note of the gay community's own racial and religious diversity, will make a good start.