I use Google Docs for creating flow charts. It's not open source, but it is free (as in beer) and does create attractive graphics. Here's an example of a simply chart
It's not very complex in that it doesn't 'know' about different types of charts - it's intended as a sandbox that you can use to create whatever chart you need. You have a wide range of shapes that you can place on the canvas, with a wide range of colours with which to fill them, easily selectable from a dropdown menu in the toolbar. It also allows you to 'snap' your shapes to an invisible line that has them aligned with each other - just place a few shapes on the canvas then start moving another one around and see what I mean.
You can download the finished images as PDF, SVG, PNG and JPG files.
You will need a Google account to use it, but you have the added bonus of not having to worry about saving them periodically, and you don't risk loosing them which, if they're for work, will be a great help.
Personally, I've never liked Dia. I've always had to fight with it to get it to do what I want, and it always made me do things in incredibly convoluted ways. The 'drawing' feature in Google Docs just feels 'right'.