Cross-linking, the practice of linking to one site from another, is a valid and useful thing to do when you own multiple websites. It is valuable because it sends both PageRank and traffic from your established sites to your newer sites. However if your sites are unrelated the cross-linking can look spammy and while you'll still get a PageRank benefit you likely won't get as much traffic through the links. Plus if you have many sites having so many external links on your pages might cheapen the look of your website, and Google (or another search engine) may think you're trying to manipulate PR and thus punish you.
So, what I recommend doing is a hub and spoke linking scheme, this type of linking is perfectly valid to do, regardless of the topics of your site, because the practice predates link popularity algorithms with search engines, and companies that care nothing about search engine optimization practice this method and do not even know it. This scheme also gives you great control over which of your websites are getting the most benefit.
The center of this method is a hub, usually a business site. All of your websites link to the hub, and then the hub links back out to your websites. My favorite place is to put the link in the copyright statement of a site. For instance my business name is Jalic LLC and so I link to Jalic.com from within the copyright statement of my websites. Even without PageRank I would still do this, as Jalic is in effect the "parent company" of the websites. Then from Jalic.com I link back out to sites I own and voila, a hub and spoke linking scheme. No matter how many sites I own each site only needs one link on it, going to Jalic.com, then Jalic.com contains a list of sites.
Well, actually, it is a little more complicated than that. You may have scrolled down to the bottom of this page and check out the copyright statement and see no link. Confused? Well the beauty of this linking scheme is that you can easily control which sites you wish to be PageRank donors and which sites you wish to be PageRank receivers. By not linking out I'm not donating any of this site's PageRank as this site is still very much in it's growth stage and I don't want to take any PageRank away from it. So if you have a site that needs PR, you don't need to link to the hub from it.
Then, on the hub, you can do many things to control the flow of PageRank. For instance lets pretend the hub has a left side menu on every page. Instead of linking to a site from a "Portfolio" or "Sites We Publish" page, you could link to it from the menu that appears on every page of the hub. This way that site would get a lions share of the PageRank and really flourish. I've gotten a new site to a PageRank of 7, initially, by linking to it in this manner. Another thing you can do is link out to subsections or subpages within a site in addition to the site itself, thus sending more PageRank (and possibly good anchor text) to that site. Of course, if you have a site which does not need any PageRank at all, you don't need to link back to it from the hub site.
If you have sites that are topically related then direct cross-links between them are still a good decision, but for publishers with a wide variety of websites the hub and spoke method is best.