Do everything you can to cross-promote them, but consider improving the experience for visitors by unifying the design, writing style, and intended audience across the network, even if the subject matter itself differs wildly.
There are successful website networks that have no apparent connection between the sites they contain, and others linked by a common theme and targeted at a particular niche. Both use similar techniques for cross-promotion, and you might like to adopt some or all of them:
Add links in the footer to all sites
Examples include the Stack Exchange network, with its related sites area (ordered by launch date):
but extend to seemingly unrelated sites, such as those in the footer of the Gawker media network:
Consider a 'site switcher' in the header
If the site network is large (or loosely related) make the switcher discreet, like Envato does for its network:
If the sites are more closely related, consider something a little more prominent that puts each site in its own tab unified across the network, like the Cheezburger abomination you mention:
Advice on 'hub' pages
If you're going to create a hub page containing all the sites you mention, make it more than just a link to the sites themselves. Consider, perhaps, a brief summary of each site and pulling in the RSS feeds to help users get straight to relevant content.
The tutsplus hub page is a fair example of this.
Consider unifying the template and design of all sites
If you're going to create a site network—even of seemingly disparate sites—it doesn't hurt to unify the look and feel. Not only does it tell visitors that they're on your network, but it enables you to share resources (like stylesheets) to reduce page load time for subsequent sites a user visits, and launch new sites faster.
Consider unifying the writing style and target audience of the sites
If you're determined to pass traffic around between multiple sites, the strategy's likely to be more successful if there's not such a marked disconnect between the writing style and intended audience of all of your sites.
It might seem crazy linking unrelated sites together, but people often have diverse enough interests to make it worthwhile. By unifying the look and feel of sites while giving each one its own personality, you can take advantage of traffic you've already captured and minimise the amount of development and marketing required when launching new projects.