Overstitch is such a useful stitch to know especially when attaching fabric on top of another piece of fabric. It can also be used to stop edges from fraying.
Basic hand embroidery stitches for beginners
How to do snake stitch / whipped running stitch / rope stitch
Zigzag stitch is a really easy decorative stitch that can be used in hand embroidery but also as a way of stitching hems and stopping edges from fraying.