A popular way is from timing events, such as a user's input into the keyboard. (It's generating the seed that is a real issue.) Using good old unreliable human input works quite nicely :)

That reminds me of some Javascript I wrote nearly 30 years ago.

I wanted to display one of 3 images randomly. So, the image was dependent on when the script loaded. 0-19 seconds past the minute for one image, 20-39 seconds for the next, etc.

On the same site, we had a gallery of nearly 100 (probably uncompressed) images - how slow must that have been on 33k dialup!!! Because I figured people wouldn't scroll to the bottom, I arranged that every hour the bottom image got moved to the top to ring the changes.