*Note: ThinkTech.ca has gone down since I started writing this :-/. Good thing I grabbed the files and posted them at the bottom of this post :).

Occasionally I look into how to create diagrams in Linux and I have either been disappointed in the complexity (BOUML) or found the solution to be too simplistic (Inkscape's connectors only have arrows as an option). Today I ran across a good solution on Canada's ThinkTek's website that allows you to express cardinality in OpenOffice Draw. I found the directions in the article titled, "Crow's Feet Arrows in OpenOffice.org" to be a bit fuzzy so I've expanded on them here.

Here's an example of the entity relationships involved in Drupal's Organic Groups:

Step 1 - Download the shapes in an ODG file

Think Tech has provided them here.

Step 2 - Open the file, right click on one of the shapes, and go to "Line"

Step 3 - Go to Arrow Styles tab, click Add, enter name for this shape

• Note the picture in the background will not change to the shape you have selected until you press OK.
  

Step 3 - In the Line options for a line created by Connector tool will now have your new definition in Arrow Styles for each end of the line

Step 4 - repeat this for each shape

I would also like to give a shout out to Creately.com for making a great online tool for just this and more: