Dia

Okay. It's nowhere near being an alternative to Microsoft Visio, but it was my initial go to when I started off consulting and wanted to quickly produce flowcharts and mock ups.

The interface was not that enticing but it's served it's purpose as a stepping stone.

Unfortunately, the Windows side has suffered for a long time. So, I have been happily using draw.io/diagramming.net. But, if you want open source then look at LibreOffice - Draw.

Light, free, simple and many features. Grid snapping can be upgraded thought.

Like that software. It's a bit unhandy sometimes but in the end maybe the best libre software there is for that purpose.

Dia has somewhat stagnated in recent years. For a long time it was the only real open source option other than turning to Inkscape and doing everything from scratch. These days I'd recommend you try Pencil instead because it has a much nicer UI and arguably better features.

I successfully made some diagrams with Dia but, as another user said, its interface is really clunky. Now I'm trying yED.

Dia on Linux - shines.

I was able to use it to create all sorts of logic diagrams - for programming and for organization. It is also capable to unite any elements and group and supports layering. Perfect software!