OS X has a built in web server. For some users, especially web developers, it may come in rather handy. You can use it for locally hosting web sites (typically for development purposes), for running a local WebDAV server (which may, for instance, come in handy for local syncing of apps like OmniFocus). Here’s how to start and stop Apache on OS X.
There are a few useful image resize apps for Mac, but one of the easiest is built right into the system. Here’s how to use it.