Old domains
Aged domain names can give your SEO efforts a big boost. You can either buy an old (aged) domain and develop it or you can buy old domains and put mini-sites (or small wordpress blogs) on them and then use those sites to link to you main money-making sites.
Old domains usually already have (aged) links pointing to them. So part of the SEO work has already been done. If you buy old domains from Godaddy's closeout auction (for $5 or $10 + renewal fee), you are essentially buying a ton of aged backlinks for a tiny fraction of what you would otherwise be paying for links.
The CloseOutReport and sites like FindAgedDomains (review and coupon can be found here) can help you save time and easily find old domains with the most SEO potential.