Ask HN: How do you back up your projects online?

I'm interested to know how people back up their code/designs. I've been using Dropbox mainly, adding the current date to the folder name (probably not the best way to do it but it works). But whenever I change something in one or a few files I have to back up the entire project directory again or replace the older file. Plus many times I actually forget what I worked on in each copy.

Do you guys know any good tools to back up stuff online where you can iterate/collaborate with others?