Sewing and helpful tools

I have been spending a lot of time thinking about sewing. Not a lot of time actually sewing, but that's the way it goes in my life most of the time. Right now I'm sewing bags for Tarot decks, and then I'll get started on making some print cotton lanyards. Not sure what will happen from there, but I have a lot of projects stacked up to get to.

Working on the bags today got me to thinking about what the most useful tool is for me in sewing. There are so many that I use and love it's hard to pick a favorite, but most of the time the thing I spend with in my hands most is this little guy here:

It doesn't look like much, but when I need to make a precise sized channel for a ribbon, or elastic, or just a nice, neat hem? It's the best thing in the world. No idea what it's called, but that's okay, it's my friend. That little red bit slides to and stays on whatever measurement I need, all I have to do is slide it up, fold the fabric over, and pin. Wonderful!!

So, what is your favorite tool for sewing?