I like @Venmo, but it makes transactions public by default (not the $, just the comment and people). It also requires everyone signs up for Venmo. If that's ok with you then it works well.
@SquareCash is nice because it's easier to use (you just connect a debit card) and doesn't feel like you're signing up for Yet Another App. It's also easier to use Square Cash from multiple bank accounts since you can just send from alternate email addresses rather than signing in/out of an app.
If you're talking about more of a roommate or closed group situation then there's @split_wise but I haven't actually used it more than downloading it and poking around.
I wrote a website for this like 9 years ago which I used successfully for 6 years. I haven't yet seen anybody make a site or app that's functionally equivalent to it though!