Diagram tools,
You an find any tool by searching from search engine such Google.
For example: usecase drawing tools, or mindmap tools.
These tools can be divided to 2 categories: online tools(run program via internet, may called cloud tools e.g. Gliffy, SpiderScribe) and stand alone tools (needs installation e.g. Smartdraw, Visio, UML tools).
You also can draw with basic tools like Paint, Word, PowerPoint