Linux

• Trimage (GUI) — similar to ImageOptim and uses many of the same lossless tools under the hood.
• ImageOptim Web Service (HTTP API) — resize and optimize images in your web applications.
• Online interface — a JPEG compressor that produces smaller, higher quality files. The web interface works with any system.

Command-line

With these tools and a bit of a glue code you can build your own image optimizer:

• pngquant — lossy PNG compressor.
• Zopfli, PNGOUT — the most efficient lossless PNG compressors.
• AdvPNG — a PNG compressor with a decent speed.
• MozJPEG — includes lossless jpegtran.
• Lossy Gifsicle — a fork of Gifsicle with heavier compression.

Windows

• Online interface — a JPEG compressor that produces smaller, higher quality files. The web interface works with any system.
• Pinga — closed source, but similar to ImageOptim.
• FileOptimizer (Download) — supports many file formats, and it's Free and Open Source.
• pngquant — lossy PNG compressor. It has some Windows-compatible GUI apps and Photoshop plug-ins available.

Older Mac OS X

If you use a Mac that is more than 6 years old, or haven't updated OS X in the last couple of years, you'll need an older version of ImageOptim:

• ImageOptim for Mac OS X 10.6 (Snow Leopard, 2009). Works on 64-bit Intel Macs only.
• ImageOptim for Mac OS X 10.5 (Leopard, 2007). Universal app that supports 32-bit Intel, and PowerPC.
• ImageOptim for Mac OS X 10.4 (Tiger, 2005).
• All other Mac versions

Mac scripts or automator

There is a command-line interface and and a Sketch plugin.