At the office I'm working behind a proxy. Is there any way to access my home computer, or just files shared there from my office?
Take a look at TeamViewer.
The client (that you would run on your work machine) can be configured to access the Internet through a proxy.
If its just about sharing files freely between two points.
SugarSync give a couple of GB storage and sync
You could also use Unite.Opera but I don't think its stable enough yet.