Full disclosure: IMPACT is a HubSpot partner -- but that's not why they're included here. IMPACT's landing pages have long been a source of design inspiration. I love the simple layout of the page, from the large headline copy and detailed featured image, to the outline that surrounds the form, to the colors and fonts that are very pleasing to the eye.
A/B testing, or A/B split testing, is a method for testing two versions of a webpage: version "A" and version "B". The goal is to test multiple versions of webpages (e.g., home page vs. product page) or one specific element that changes between variation A and variation B (such as having a lead form on the left hand side or having it placed on the right hand side), FAQ to determine which version is most appealing/effective. This testing method may also be known as A/B/n split testing; the n denoting more than 2 tests being measured and compared. The data for A/B testing is usually measured via click-through rate or an alternative conversion tracking method.