To present a sample of your product, just show how it works. There are two formats of files could do this online. Video or GIF. To make a quick and short screen records, GIF could be more convenient as the file size is smaller and can be easily embed as an image while the video fit for high quality screenshot that need audio support.
Here are the apps that I found that could be an assistant to easily record the screen and save the record as a GIF.
With GIF & Twitter support
Records Screen and save as Gif app. Open Source.
supports GIF & video. Share screen captures in seconds. Support Windows, Mac and Linux.
Cropper is a screen capture utility written in C#. It makes it fast and easy to grab parts of your screen. Use it to easily crop out sections of vector graphic files such as Fireworks without having to flatten the files or open in a new editor. Use it to easily capture parts of a web site, including text and images. It's also great for writing documentation that needs images of your application or web site. Open Source.