I'm a heavy Reaper user since I love the experience of editing with it, but when I'm fooling around writing songs, I use Garage Band specifically because it has so many great instruments and sounds, and also because I find I interact with it much differently than with Reaper or Pro Tools because of the simplified interface and I don't get sucked into fiddling with the details of what I'm making.

Before Garage Band, I used Tracktion (I think its now called Tracktion Waveform Free) in the same manner. It's been ages since I used it but if you're a Windows or Ubuntu user I think it'd be worth checking out.