A free online UI animation tool can mean three different things: a place to prototype a transition, a place to make a playback asset, or a tool that produces motion your app can run. Free plans exist in all three categories, but they do not give you the same output.
For quick prototypes, Figma and Jitter are the usual starting points. Figma's Smart Animate can tween between frames, and Jitter is built for short interface animations. Both run in a browser, and both have free ways to start. They are good for agreeing on timing and feel before code. Neither is a runtime that ships a state machine in your app.
For playback assets, LottieFiles is the common free entry point. You can preview and test Lottie files, and the format is supported on web, iOS, Android and Flutter. A Lottie file plays a fixed animation. It does not decide what to do when a user taps, fails a form or changes a setting.
For interactive production UI, ExodeUI's Community plan is free. The editor runs in a browser or as a desktop app, and the output is one open .exode JSON file played by runtime packages for React, Vue, Svelte, Angular, Solid, React Native, SwiftUI and Flutter. State machines, spring physics, inputs and events are part of the file. Professional is $29 per month. ExodeUI is newer than the established tools, so test the runtime path you need before committing a team.
Free is not the same as unrestricted. Check whether exports carry a watermark or splash screen, whether commercial use is allowed on the free tier, how many seats you get, and what happens to files if you stop paying. Those terms matter more than the sticker price.