I am a one-computer shop, and increasingly I am thinking I need some redundancy for storing programs and other code.
Should I use code.google.com, or sf.net, or codeplex.com or another online Source Control system for this? It wouldn't be to host a particular project, but more like a storage area for everything that isn't in a big project.
I think rather than a project-hosting site, something like Github or Bitbucket would be more appropriate. Its definitely practical to use them as an off-site backup system for code.
Project hosting sites, especially OSS ones, don't particularly appreciate being used for personal purposes only. (If you have an SF.net account and develop open-source with your account, then I think they'd be fine with you using their dev hosting.)
Just choose a reputable one which won't disappear overnight!
If you don't want your code and/or to be publically accessible (even if just by accident), you might want to consider renting a virtual server and rolling your own SVN/GIT/... server.
Keep in mind that the free option limits you to public Git repositories on Github.
Bitbucket offers a single private Mercurial repository (up to 150MB) with the free account option.
ProjectLocker is a free Git (and SubVersion and Trac) hosting provider but, I should point out, the security of their system has been called into question here although they seem to have addressed the question.
For that reason, going with a more established provider or paying a small monthly fee may be a more desirable option if the security and privacy of your project data may be an issue.
It is probably a good idea to store you stuff somewhere else (ie. either an external hard-disk or online).
If you want the features of source control, then you need an online source control repository. There are plenty of them around, some free, some paid for. Assembla, GitHub, Unfuddle and Devja Vu are some of the common ones. I use the free version of Beanstalk personally and find it very useful.
If you just want some off-site storage, then look at sites like DropBox (although DropBox does allow versioning of files), or even Microsoft Live which provides several GB of disk space.
Whilst you can pay for most/all of these services, there is usually a free version to get started with which has lower storage space or less features. These free accounts are good to try-before-you-buy if you really need that extra storage space or features. It pays to test a few sites to see which fits what you need first.