There are no free tools that compare to Photoshop, I recommend saving up for Photoshop Elements, it's like 100$ these days.
Oh noes! Lack of proper CMYK and and a hard limit of 8-bits per channel!
Beyond that, though, the two are pretty much feature-identical, just a matter of knowing how to access those features, which is the biggest challenge for either is learning the UI. Once you've learned the UI for one, switching to the other is like learning from the ground up all over again.
Of course for professionals, lack of proper CMYK and forced to work with a maximum 8-bits per channel is a big deal, but for hobbyists, its meerely a matter of which did you start using first as that is obviously going to be the one you are most comfortable using.
I use the brush tool and hold down 'Shift' or 'Control'; whichever will allow me to make straight lines. That, or I create paths and fill them. Works just as well.
Yup, this is the simplest way to do it IMO, the alternative is to use the path tool and then click "stroke path"