Free Video Watermark is an extremely basic tool for adding image or text watermarks to your movies.
The program's main advantage is its extreme simplicity. There are barely any options: enter some text, choose a font and colour, or open an image file, and that's essentially it. Your text or image is overlaid on a target video and exported to the folder you specify.
By default the text or image are displayed top-left, but you don't have to live with that. If you prefer your watermark centred, or bottom right, just drag it there with the mouse.
What you can't do is set the transparency of your object, a particular issue with images. They're not so much watermarks as overlays, just dropped on top of the movie.
Free Video Watermark is basic but undeniably easy to use. Worth a look if you only have occasional, simple watermarking needs.