TinyPNG

TinyPNG is one of the most popular free online image compressors. Drop in a PNG or JPEG and it uses smart lossy compression to shrink the file size, often by 50-70%, with almost no visible loss in quality. It is perfect for speeding up websites and saving storage.

The web app is free for images up to a generous size limit and needs no signup. There is also a paid Pro plan and an API for developers who want to compress images in bulk or automate the process inside their own apps.