Chemix is a free portable flash tool that lets user create diagrams of lab equipments and lab setup of Science(both Chemistry and Physics) experiments. It is specially geared towards students, since it is not a professional tool, it can be used for making quick presentations.
Usability is a piece of cake, just select an item from the top toolbar and drag them to the workspace. There are a total of 5 different categories with 8 items per each. To change the category, click the smaller unlabeled buttons on each side of the toolbar.
You can rotate the object, edit it, and delete it(by moving to Bin). By default the objects will be in the air, click the button on the bottom-left side to bring the table. Now drag the objects and place them on the table.
Sadly, there is no build-in option to save the diagram. You will have to take the screenshot, paste it in Paint editor, crop out all parts except the diagram, and finally save it in Jpeg, Png, Gif or any other image format of your choice. It is available for both Windows and Mac OS. Enjoy!