Tim Berners-Lee gave the British Computer Society's 2007 Lovelace Lecture last night at the Congress Centre, on Great Russell Street in London.

It was free to attend (you just had to book in advance) and it was only about 15 minutes walk from where I work. The lecture content was similar to some of his other lectures I've seen webcasts of, but it was worthwhile going to a lecture in person, even if the majority of the audience weren't quite the young Web coding types I'd expected.

Update:
Lecture slides (Thanks Jean-Gui!)
Lecture on Google Video