In a month's time, I'm travelling to the United States to speak at a conference in Jacksonville (unpaid). I don't know if I need a special visa for this, as the border agency website doesn't have info on this.
I won't be paid for it and will mix in a bit of tourism, but don't know if my ESTA will allow me entry.
This is actually very clearly covered by this document published by the Department of State, which is linked from the page on the Visa Waiver Program:
Conference, meeting, trade show, or business event attendee: Will receive no salary or income from a U.S based company/entity. For scientific, educational, professional or business purposes.
Lecturer or speaker: No salary or income from a U.S. based company/entity, other than expenses incidental to the visit. If honorarium will be received, activities can last no longer than nine days at any single institution or organization; payment must be offered by an institution or organization described in INA 212(q); honorarium is for services conducted for the benefit of the institution or entity; and visa applicant will not have accepted such payment or expenses from more than five institutions or organizations over the last six months.
As you are going there unpaid, this is clearly within the activities allowed by a Visitor (B) Visa, and thus for the VWP.
