If you're a web developer, you want your web apps to load fast and be easy on your visitors/users data plan or bandwidth. A utility like YSlow will tell you that you should minimize the number of requests required to load a page, in addition to keeping the responses as small as possible. A common way to accomplish this is to combine, minify and/or compress non-dynamic content like stylesheets, external Javascripts and background images.
In the .NET part of the world there are many solutions to this problem - libraries that plug into you app and help you define bundles of files that can be combined and minified, helpers for spriting background images and so on - but none that I've found so far is anywhere near as simple to use as the awesome RequestReduce by Matt Wrock.
RequestReduce is a single NuGet package (two if you want SASS, LESS and CoffeeScript support, three if you need to deploy your app to a web farm) that hooks into the application, magically determines which style and script files can be combined into bundles, minifies these bundles, creates sprites for background images and finally replaces the original file references with the optimized versions in the HTML output. Pure genius.
A tip though: it won't update the cached optimized files unless the original content url changes. Therefore you should use some kind of versioning strategy for your static content, like including a revision number in the filename or appending a timestamp to the querystring of the content url (see this snippet for a sample UrlHelper extension that makes this dead simple).