The most popular, in my unscientific observations, is Audacity. It’s free and it’s capable, but it leaves a lot to be desired for it’s usability. Among Mac users, Garageband is popular, again because it’s capable and free, but it’s ease of use leaves a lot to be desired.
If these are fine for you, great. Keep using what works for you. However, if you want to step up to a bigger and more professional tool, there are options. Your biggest one is Pro Tools.
Pro Tools has been around for quite a while, changed hands several times, and changed it’s philosophy in large and small ways over the years (the ability to use any hardware several years ago was a major change for them), and still it remains the industry standard. If you walk into a radio station that does it’s own production, chances are, they are using Pro Tools. If you are in a recording studio, they most likely have Pro Tools. If you want to work in audio beyond your own home studio, it helps to know Pro Tools.
All of this is a long winded way of pointing to some excellent tutorials about Pro Tools from the fine folks at Transom.org. They recently revamped their Pro Tools tutorials, updating them for the more recent versions of the software, and focus on the needs of the radio and podcast producer, rather than the music producer.
Looking these over, I want to do something similar for my preferred DAW, Logic. I find it easy, cheaper, and just as powerful as Pro Tools, and think most users would benefit from it over the scaled back programs like Audacity and Garageband.
If you are interested in using more professional tools than what you already have, Pro Tools is a great choice. There are other options out there, and I’m going to lobby for some of them.