I don't know if this will end up being a frequently updated list, but there are at least two things which are on my to-do list, to be done eventually:
Interactive EA flowchart
People can choose what they think they want to fund, earlier or later, higher probability of success or lower, etc. etc. and then receive a charity/philanthropic recommendation. Useful for getting new people into EA?
ALREADY CREATED. See here
Carbon Emissions Calculator
An interactive web-app where users can see projections for future carbon emissions, conditional on them taking certain actions. Also what happens if they convince their town, country, or the entire globe to follow suit.
Social Networks for Blogs
Some sort of open protocol that abstracts the benefits afforded by social networks like Twitter or Facebook–i.e. followers, comments, centralized feed, sorting by user–except on the level of individual websits. Something like Wordpress or Disqus, except less bloated, more customizable, and no ads.
Effect Size Calculator
An interactive graph that shows how Cohen's d relates to correlation coefficients. You'd be able to change the mean between groups, as well as the difference in variances.