Googler, JohnMu, has some basic linking advice in a Google Groups thread. In short, John explains that logic behind who you should or should not link to. I'll quote John:
In general, if you can provide relevant links for users of your site, feel free to link to them. That's how the web works :-), even if those links partially come from sites that you also own or work on.
That said, if those links are not relevant to the site, it's generally safe to say that they will not provide any significant value to your users, even if the sites you're linking to happen to be owned, made or run by the same owner. If you own or have made a group of fairly unrelated sites, it might be best to keep them on a portfolio page.
In general, thinking about where and how you would link to these sites will provide a good guideline. Would you want to keep this link visible in your main content? Or would you prefer to keep it in 5pt light-grey on white text on the bottom of your pages?
Did we learn anything new from this? Not most of you, but for our new readers or those searching for "advice on linking out," this article may come in handy.
If you have more questions, join the thread at Google Groups.
Note: This article was written last week and scheduled to be published automatically on Monday, April 21st.