Best audio editor: Audacity

Audacity isn't the most advanced editor around, but it's absolutely perfect for tweaking audio levels, reducing hiss and noise, and performing basic equalisation thanks to its wide range of built-in effects. You can use it to convert audio between formats, and record directly into it – with a live waveform display which is handy for keeping an eye on your levels.
Audacity's key selling point – as if it needed one, being free – is its multi-track editing interface, which is perfect for cutting and splicing together podcasts and such. Do be warned, however, that this app requires negotiating a bit of a learning curve to get the most out of it.