I find Paint.NET is one of the easiest graphic programs out there, and it's free too. It shows transparency by default, but doesn't let you edit it, however there's a transparency plugin that lets you adjust the transparency for any selection you make.
Paint.NET:
http://www.paint.net/
The transparency plug in:
http://paintdotnet.forumer.com/view...&st=0&sk=t&sd=a
I find that's a lot easier than working with a separate alpha channel, it's much easier to just make a selection and choose to have it transparent, not transparent, or anything in between.