Diagramming tools

Interesting tools for building diagrams:

DSL based

These tools take as input a text file with a description of the diagram to produce and generate an image as output.

Graphviz [official web]
Simple but powerfull tool for graph visualization
PlantUML [official web]
Tool to build UML diagrams, wireframes, Gantt diagrams, mind maps, ...
Ditaa [official web]
Build general diagrams from ascii-art.
Mermaid [official web]
Visualize graphs, sequence, Gantt, class and git diagrams.

Tools to build diagrams through drag & drop.

Interesting software to build presentations:

mdp [on GitHub]
A command-line based markdown presentation tool.
emacs + org-mode + reveal mode [on GitHub] :: A tool to convert org-mode files into reveal.js HTML presentations.

Interesting tools for text analysis:

This is software that has stood the test of time, and, though it may seem antiquated, it’s as robust as an old wise oak tree.