A microsite, also known as a minisite or weblet, is an Internet web design term referring to an individual web page or cluster of pages which are meant to function as an auxiliary supplement to a primary website… Microsites may be used for purely commercial purposes to create in-depth information about a particular product, service or as editorial support towards a specific product, such as describing a new technology. (
Wikipedia)
A Microsite design is an online marketing experience that generally highlights a particular promotion, new product, or seasonal event. For example, a Microsite design or Flash microsites could be used for the launch of a new car or beverage.
Below is the some microsites designs examples.

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Microsites Designs Examples
Microsites Designs Examples