Here’s a fun application: C64 yourself.
Drop an image (PNG or JPG up to 2 MB in size) onto the website and it will convert it to a Commodore 64-8-bit style image. It has all sorts of retro loader-screen loveliness while it does it.
You can then save the image in three resolutions: 320 x 200, 640 x 400 or 1024 x 800.