MightyGif is a small set of GIF tools that do one thing each and do it in your browser. You pick a file, the page does the work on your own device, and you download the result. Nothing is uploaded, so there is no queue, no file size you have to wait for and no copy of your file left on someone else's server.
It is one of the Mighty projects by Steve Sicherman, alongside MightyWidgets and MightyBuilder. The tools are free and carry no watermark.
What runs where
Decoding, resizing and encoding all happen in JavaScript inside the page, using a background worker so the tab stays responsive. The only thing the site loads from elsewhere is Google Analytics, which counts visits. It never sees your files.