Effective API Design presentation by Joshua Bloch
Although from JavaPolis 2005 this is still a very nice presentation by one of the gurus in the field of API design, Joshua Bloch. The presentation runs about 70 minutes but is well worth it.
Summary: "A well-written API can be a great asset to the organization that wrote it and to all that use it. Given the importance of good API design, surprisingly little has been written on the subject. In this talk (recorded at Javapolis), Java library designer Joshua Bloch teaches how to design good APIs, with many examples of what good and bad APIs look like."