It's called Explore2fs. It's available on this link:
As there's very little sunlight between Wubi's root.disk and swap.disk files and genuine Ext2 filesystems/partition files, this app is even recommended by the folks at Wubi as a file transfer solution.
I've been using Wubi for my Ubuntu/Kubuntu installs since last October, at least, and as the .disk files at C:\Ubuntu\disks\ are visible, though you have to deselect "EXT 2 Image files" and select "All files" in the Open dialog to see them. Explore2FS starts you off, when you do so and open root.disk, at the topmost level, and you can navigate much as you would with your folders visible in Win Explorer, to /home, /bin, /usr, /etc, wherever you have to look to find the file(s) you're looking to extract. Then it's a right-click and selecting the command "Extract file(s)" that does it; it brings up an ordinary Windows "Save" dialog from which you can navigate to the folder into which you wish to extract your file/s.
There's more on the developer's site.
Like bobc, I also keep a dropbox for when I'm booted into Kubuntu. I was a Mac OS X home user for five years, and was always annoyed that, in the OS versions I was using, the folder at /Users/yourname/Public/ was only reliably accessible for read-write via ftp or ssh or something on that order. So I laid out my dropbox path as "Users/myname/Public/" as a sort of "Take That, Cupertino!"