Recently I've been having a play with the Box.Net API and have built a C# class to wrap the API in. I'm planning to release the source to it here, but there's still a little work to be done on it yet.

Using that API class I'm also spending a little time, whenever I get a chance, on Box.Net Explorer. Box.Net Explorer is an app I'm building that lets you browse your Box.Net account as if it were a local folder on your Windows PC. It then let's you download/upload to your account via copy+paste or drag/drop. The most important thing about Box.Net Explorer is that it integrates into Windows Explorer - so there's no other app involved. You just go into My Computer as you normally would and you'll see a Box.Net special system folder.
Think of FTP, but tailored for your Box.Net account, the BNExplorer will cache your folder/file list, so browsing it is very quick, but should you need to refresh the view you can double-F5 and it will update the list.
You will be able to (when it's finished) drag files off of BNExplorer to any other local folder, and a special downloader will appear and show you the progress of your download. Likewise for uploading.

You're probably thinking that this doesn't offer much benefit over FTP access at the moment, but here's where it gets interesting:
Because BNExplorer uses the Box.Net API, it also gives you access to each individual file/folder's settings, so you can add/remove tags, configure sharing and other features unique to Box.Net.

I've not got a massive amount of time to work on it right now, but I'll hopefully have a preview ready for early next week. But I'm not promising anything!

Here's a few screen shots of what it looks like so far:

This is how it appears in my "My Computer". It's a system folder, so very similar to the Control Panel in that it's entirely virtual.

This shows downloading from BNExplorer to the clipboard. It doesn't support drag and drop right now, so this was acheived by right-clicking 'Copy'. This isn't very useful, but it's a very early build.

This shows the top level "MyBox" of my Box.Net account. I got here by clicking the Box.Net icon in "My Computer"

I have a business model for this app, and for the Mac version I'm planning, so I won't be releasing the source code for it. It will be a free app though. It's going to serve as a demo for a larger project that I'm involved in right now that involves adding functionality to web applications through tailored desktop applications for Windows/OS X. But more about that later...

Update: Well, it appears David @ BoxDrive is doing essentially the same thing! Great minds... (except he thought of it a few weeks earlier it would seem :)