I've used GnuCash, but the UI is very ugly (programmer design—disclaimer: I'm a programmer) and it doesn't work over the web.
At the moment I use waveapps.com. It's not open source, but it's free. From the outside, it looks like it's for businesses, but once you create an account, on the top-left you can toggle between business and personal!
At the moment I use waveapps.com. It's not open source, but it's free. From the outside, it looks like it's for businesses, but once you create an account, on the top-left you can toggle between business and personal!