I had an invite to Box.net yesterday from a friend so I went and had a look to see what it was all about.
Basically, it's an online storage type thing with 1gb of free space similar to Flickr but not restricted to pictures. I used to use something similar a long time ago which disappeared off the web back in the 90's.
There's an upgraded version with extra features and a 5gb limit which is $5 a month.
The free version gives you 1gb of space, drag & drop and public/private files. You can't deep link files using the free version but you can share them with a url which will serve a page with a thumbnail link.
Anyone can register for Box.net and there is an affiliate-type scheme where you can get a free upgrade by referring 5 people to their service.