Simple pure sine-wave test source...

[Diabolical Artificer]

There are several free bits of software that have very good low THD sig gen's in them like Audacity, REW, Alta and Soundcard Scope. The latter is a easy to use and is very useful as it has a scope, FFT + THD measurement, etc. Some of these are quite complex and take a bit of sussing out, but you don't need to get your soldering iron out.

Andy.