Freeware is a plus. Could you also describe with one sentence why it should be good enough for occasional usage?
Edit: You might check this cool link , providing more info about entity relationship diagrams and tools
Edit: For the chosen answer couple of tips - click on entity even it looks like circle and type the name and Enter - it will give you the Entity box - Click on association , Ctrl + L would give you the nice line
You can use StarUML with the ER extension. It is totally free, stable and easy to learn.
Update: 09.10.2017: StarUML is not free anymore!