This is a pretty old flash-based resource hosted on STEM.org.uk (requires a log-in but it’s free). It is still my favourite tool for practising solving linear equations. I’m worried that I haven’t found anything better yet and that as Flash become less and less supported, I’m not going to be able to use it much longer. Let me know if you know of others!
I use it to really reinforce the idea of “doing the same to both sides”. It can be used both in whole class explanations / discussions and also with students using it themselves. The process of “getting it wrong” is really productive as they can see what would happen if they did that. I also like the fact that there is no “undo” button. They need to work out that “undo” just means do the inverse operation so it reinforces that idea too.