Hello :)
I'd like to host some open source code on a public repository, and I really like how pretty GitHub makes this kind of sharing.
Unfortunately, Git is not an option for these projects, for the most part because I'm developing for/on Windows, and Git support on Windows is not anywhere near where Subversion's support is.
I'd like to have something exactly like GitHub, but one where I could use Subversion instead. Does such a service exist?
I use Google Project Hosting with Subversion. I think it's a bit more involved than GitHub, but it provides free hosting for Open Source projects.
I've used http://beanstalkapp.com/ and found it to be good for hosting projects. You should definitely give them a look if you really have to stick with SVN.
Sourceforge offers SVN as one of many features, and is free for Open Source projects, as does Google Code
http://unfuddle.com/ is one SVN host I have no complaints about at all.
I'd like to have something exactly like GitHub, but one where I could use Subversion instead. Does such a service exist?
No, it doesn't exist.
And it cannot exist. The features of GitHub are highly dependent on distributed version control. In fact, the majority of all the features that make "sharing pretty" have absolutely nothing to do with GitHub, they are fundamental features of distributed version control.
So, you can have something exactly like GitHub (or Gitorious, codaset) for Mercurial (BitBucket), darcs (Patch-Tag), Bazaar (Launchpad), Codeville, Arch, ArX, Monotone, Fossil, but not for Subversion (or CVS, Perforce, Visual SourceSafe, Team Foundation Server, Vault), because centralized version control systems simply are not capable of supporting the required features.
There is http://svnhub.com/ — operated by the people behind github. Seriously, use git!