Some time ago, creating different domain names and linking them together could generate large number of inbound links to your main website and increase its SERP ranking. However, in the era of penalizing search engine algorithms such as Penguin 2.0, you have to think twice before attempting to do something like this. Linking the various sites in a network together or simply linking domains can now look spammy and attract the wrong kind of attention. In a GoogleWebmasterHelp video, Matt Cutts answered the question as to whether it was possible to link 20 domains together without getting into trouble.
Domain Linking: Best Practices
According to Matt Cutts, it is not a right thing to link 20 domains together if they discuss the same topic. For example, it is better not link 20 domains if they are all talking about the same thing. For instance, linking several domain names for patio-furniture-sets or cheap-Indian-artificial-jewelry could end up as spam. Cutts suggests the following practices for linking domains without getting into trouble:
Link the domain names only if they are different versions of your domain in various countries (For example, Google.co.za (Google version South Africa), Google.fr (Google version France)). Different versions of the domain make sense here.
Do not link to large number of domain names in your footer as they will look spammy.
Have one link to the country locator page from where readers can choose from your different domain versions. Place flags or dropdowns for readers to pick the country of their choice to get to those domains.
Ensure that all links to your domain versions are static HTML links so that Googlebots can follow them and transfer PageRank between them.
Linking in Large Blog Networks
As you know, large blog networks link several blogs together. In such cases, it is possible to find large number of footer links to multiple unrelated blogs. Cutts says that even if you have 10 domains (of blogs) in the network and only few posts in each of them, it is not a good practice to crosslink them together unless you have a solid reason to do so (such as a location issue).
Penguin 2.0 strictly curbs massive cross-linking practices and considers them link spam. Erring websites are penalized. Matt Cutts says that even if you are linking large number of domains together without the intention of cross-linking, Google could view it as a massive cross-linking scheme.