HelpSpace offers a really affordable and nice way maintain good relations with your customers.
I've tried other deals on AppSumo but I really like how HelpSpace has implemented its main features.
- Shared Inbox
- Rules
- Self-service sites (knowledgebase, documentation, or even blog)
It's really easy to setup and get up (with custom domain) and get running in a few minutes.

The good:
- Easy to setup
- Great support experience
- The article editor is really good (and simple to use and offers a great editing experience).
- Self-service sites with multiple templates (basically now I'm running my blog on HelpSpace)

Things that I like to see:
- The possibility to edit alt attribute for images
- API (in general to push clients from other systems)
- The possibility to toggle on/off the search functionality for the Blog template