For simple vector diagrams, I've found https://www.draw.io (similar to MS Visio) to be very convenient. The code is also open source (Apache-2.0): https://github.com/jgraph/draw.io
[1] https://www.yworks.com/products/yed
[2] http://yed.yworks.com/support/qa/515/why-not-an-open-source-...