It was paid for by the NIAC, the outfit run by prominent Iranian-American author and pundit Trita Parsi. From their press release:

“The U.S. and its closest allies are on the brink of a historic deal that will both prevent an Iranian bomb and war with Iran, and Congressional hawks are orchestrating political stunts with foreign leaders to try to kill it,” said NIAC President Trita Parsi. “The American people do not want another senseless military adventure and certainly don’t consider Benjamin Netanyahu to be their commander in chief.”

...in one of the most critical national security debates of our time – the decision of whether to invade Iraq – Netanyahu was brought to testify before Congress. In his remarks he advocated strongly for the war, telling lawmakers ‘if you take out Saddam’s regime, I guarantee you it will have enormous positive reverberations on the region.’

The statement also reminds us of this episode:According to Jewish Voice for Peace, as of today, 29 members of Congress will skip Netanyahu's speech. You can sign their petition here

Or, you can contact your members of Congress directly and tell them to #SkipTheSpeech:

Senate

House of Representatives

UPDATE: Here's the Hill's 10-day-old list of those who have stated publicly they will skip the speech: