Anyone know how to build a menu like this?
Welcome to stackexchange. When asking a question please try to be as specific in your question as possible. Try describing what you want to know – also using screenshots or wireframes is ok – but please avoid forcing others to ckick on a link to understand your question. Questions like "How can I do &lt;link&gt;this&lt;/link&gt;?" are usually not very likely to get a lot of attention from the community. 

That said – I did click the link – and I suppose whether there is a module providing this effect/functionality or not is probably not that important since you could achieve a fairly similar effect with either a drupal menu or a drupal view and some javascript / jquery / ajax (for loading the second level dynamically) and some additional css3 for the sliding effect. 

You could probably even skip the scripting part, display a standard list of links, hide the non-active second level items and use css3 to slide the second level in once it's needed. Asking ofer at stackexchange might be the more suitable forum to ask about this (here is for example everything about 'slidetoggle').

I'm sorry this is probably not the answer you were looking for – but maybe if you provide more information on what you want to achieve someone else can point you to an appropriate module.