To create diagrams, I a few different tools.
Lucidchart - basically the closest I can get to Visio whilst using a mac. Yes, I like it so much that I pay for it. I use Lucidchart for:
(yes, I would like to use one of the tools that integrates with Confluence, but they just don't cut it with some of the visio-like features of layout and snapping).
(yes, you can do sequence diagrams in Lucidchart, but why, when there are much easier ways to create them - automatically).
Graphviz is the tool of choice for flowcharts - see my tutorial about using it in Confluence (unless you use Confluence On Demand).
For sketching out flowcharts on the iPad, I LOVE LOVE LOVE Instaviz
Balsamiq for Mockups and Wireframes where necessary (yep, paid for that too).
For prototyping quickly on the iPad I love Adobe Proto (yep, bought that one too, but it is no longer updated).
Sequence diagrams for quickly and easily documenting a processs.
For sequence diagrams you can use.
But I use Plant UML and the excellent plugin for Sublime Text (yep, I paid for Sublime Text too - see MavensMate and Git for Non Developers as to how I use Sublime Text for Salesforce work).
Why I like sequence diagrams and the setup I use:
And no, I am NOT strict about my UML conventions (see the diagram I made for Building a "Simple" Case Management Process), just enough to show the client what is possible.