Currently I work on visualizing the health of components, nodes and web-services. Therefore respective components expose their status with a health-check. For a Restful Webservice a simple ping request (simple GET with Status-code 200) is ok. A more advanced technique would be response with a brief summary of status and version. This status I want to illustrate with an appropriate symbol.
My sole requirement: It has to be simple. I was thinking about using Font Awesome, but therefore you need to include the library. I found this beautiful webpage - http://www.sabinanore.com/design/html-special-symbols/ - , which gives me exactly what I needed and more. I can use following HTML codes, that are also compatible with markdown. I have added the link to http://graphemica.com/, with additional information how that they can also be used in Python, Ruby, Java, etc. If you need to use different color, to distinguish the status better, I added the green symbol.