There are various ways to show off your designs and prototypes to clients on the web, however usually they require you to go through the hassle of saving your PSD as an image, and then uploading it to somewhere. When you make little changes, saving and uploading designs ends up becoming a tedious process.
Marvel is a web app that makes showing off designs to your clients a whole lot easier and quicker - for both the yourself and the client.
Marvel links directly to your Dropbox. You can then choose a folder to sync with. Now instead of saving images, you just save the PSDs. Marvel will then take the PSD and turn it into an image itself without you having to do anything. You can share the link with the client so they can see the PSD. When you make any changes on the PSD, you simply save them and it'll update them in real time. So your client doesn't have to refresh or open any new links when an update is made - they'll see the changes right there in front of them.
The best thing about Marvel is it's free. The UI is of the web app is absolutely fantastic and the developers are continuing to build on it to make it better and better.