On the daily work, we might need to draw out lots of diagram, such as flow chart, network, organation chart.
Usually we can use Visio, Excel to assit on this. But that do have a lot of online tools today allow you to do it online without any installation.
The tools that I recommend is CACOO , it offer range of diagram to select.
Another cool things that I always use is, share. You can share the diagram among your friend.
So if you looking the simple diagram drawing tools, you can consider this. You might use this link to register the account, I can earn some extra sheets.