It's no surprise Unbounce is near the top of this list -- they've actually written the book on creating high-converting landing pages. Although there are lots of amazing things about this landing page, the two that I absolutely love are: 1) The use of a chat window instead of a classic form, and 2) the detailed -- but well packaged -- information below the form.
Testing can be conducted sequentially or in parallel. In sequential testing, often the easiest to implement, the various versions of the webpages are made available online for a specified time period. In parallel (split) testing, both versions are made available, and the traffic is divided between the two. The results of sequential split testing can be skewed by differing time periods and traffic patterns in which the different tests are run.
I'm not sure how the algorithm works (or if there's one at all), but while I was filling it out, I had some anxiety about not qualifying. Once I found out I did, I was excited to fill out the form, which I'm sure most people who are in debt and using this tool are. By making this offer seem more exclusive before the form appeared on the landing page, I'd bet that Bills.com increased conversions pretty significantly.