Yesterday I spent a few hours building an elaborate web photo gallery for our wedding photos before my wife and I realized we're more interested in seeing everyone's likes and comments than in building yet another elaborate web thing for our wedding. (That's to say we do value building nice web things, we just value getting Facebook love from our friends more.)
The photo gallery I made is modeled after Facebook's newest lightbox UI for browsing and viewing photos, which has some nice details that I love, such as pushState support so browsing between photos doesn't break your browser's back button. But the best one is how they handle scrolling: you can scroll the contents of a photo lightbox (which can include comments, likes, and other metadata that can make the viewer taller than your browser window) while preserving your scroll position on the photo grid underneath.
I was afraid I wouldn't be able to figure out whatever strange web magic they used to create this effect. But it turns out Facebook's own frontend dev team posted a detailed explanation of how it all works, and it's beautifully simple. In fact, almost all the important layout and scrolling magic is handled with just CSS. Really cool.