Yes, Microsites are common among corporates in the UK for presenting Brands or Services with their own identity, but hanging under the corporate umbrella-site or running on a third party site. They are often marketed independently but more often than not, follow corporate website guidelines.
These days, in SEO terms, I am guessing that there are some good reasons why a “Microsite” is useful (Gary will no doubt be able to help here), although a key facet was to ensure that “detached” content is still seen by the user to be “owned” by the main brand.
One good example might be a financial services site hosting Microsites for different financial service providers/brokers they deal with.