I recommend https://draw.io/ - it's easy to use, doesn't require an account and you can load/save diagrams from Google Drive, OneDrive and Dropbox (or, of course, from your device). I've created a lot of diagrams with it and it "just works" :)

Seconding draw.io, have been using it somewhat actively recently, do recommend.

Seamless integration with GDrive, lots of existing templates, great experience overall.

Most of the time I'm using PlantUML [1], since it can live in the same place as the source code, moreover there is a IntelliJ Idea plugin which adds PlantUML support to the IDE [2], nevertheless I think there should be plugins available for all major IDEs.

[1] http://plantuml.com/

[2] https://plugins.jetbrains.com/plugin/?idea&id=7017

=> https://gephi.org/ + log aggregation/tracing - see http://opentracing.io/ and http://zipkin.io/

I like Lucidchart (http://lucidchart.com).

Does what I need, and their google-drive integration makes it easy to keep track of the diagrams and share them with folks.

I use either https://draw.io or Giffy Diagrams for Chrome extension.

libreoffice draw works fine for most of my charting needs. even google drawings is not that bad. draw.io is plain awesome!

www.yuml.me is text based, which I prefer