I use the gimp right now, and sadly, making good proton streams is no easy task. That's really all I've been using it for since I got it, and it's just not as easy as it is in photoshop. There's no glow option for the layers. I'll see what I can whip up and I'll post it here with a tutorial for any gimp users.
Ok... so a tutorial on this was pretty hard. It was difficult for me to keep up with what I was doing, but I remember the most important bits!!
After like, 20 minutes (in photoshop, this can be done in about 2) I got this. I think this is as close as a non-pro in Gimp can do.
You just go about your normal business, making a new layer, draw your beam blah blah. The most important part to get is the glow. You have to first duplicate the beam layer of just the beam, then go under "Script-Fu" and use the Alien Glow feature. Then, after setting your colors, you need to delete the once duplicated layer and the new layer titled "background#1" there. You have your glow. Now, you're going to need to duplicate that layer a few times to make it noticeable. Keep in mind, set most layers here to "screen" to give it that glowy look. Your colors are going to wash out, so you want to modify the colors to be more orange. Go under tools>color tools and use what you've got. A little motion blur, some opacity and layer adjustments and you get that. It's not that great, but it's better than paint, IMHO.