Ask HN: What tools do YC co's use to create beautiful landing pages quickly?

Most YC companies tend to have beautifully polished landing pages by demo day [1]

What are some useful templates/libraries to build similarly polished landing pages (i.e. significantly better than hacking together a quick Twitter Bootstrap page) in a relatively short amount of time with limited resources?

[1] Look at http://www.ycombinator.com/companies/ for some examples