A question that comes up very often is what activities can conventional testers engage in on agile projects? To me, the key to testing on any project is to provide feedback. On agile projects, the code should always be available to test, and iterations are often quite short. Therefore, a tester on an agile project should perform activities that provide rapid, relevant feedback to the developers and the business stakeholders.
A tester needs to provide feedback to developers to help them gain more confidence in their code, and (as James Bach has pointed out to me), feedback to the customer to help them gain more confidence in the product that is being delivered. So an agile tester should identify areas to enhance and complement the testing that is already being done on the project to that end.
There are potentially some activities that are more helpful than others, but providing feedback is central to what testers do. Some feedback may be in the form of bug reports, or it may be a confirmation that the code satisfies a story, or the application works in a business context.