The most natural approach to testing in an agile environment is in my opinion exploratory testing http://en.wikipedia.org/wiki/Exploratory_testing.
Doesn't sound words like
According to Cem Kaner & James Bach, exploratory testing is more a [mindset] or "...a way of thinking about testing" than a methodology
or
pair testing
sound familiar to agile developpers. Testers can be involved much earlier in the process than in traditional testing.
