When I'm doing documentation, I frequently want to build a small ERD by hand to demonstrate a couple of relationships that are associated with the current topic.
I can do it in Visio, and frequently do, but it's a pain in the neck sometimes. Visio has rules in place that are sometimes difficult to configure and sometimes the line endings are painful to change. The plus side to using Visio as opposed to anything else that allows you to quickly draw boxes is that the line endings can be used to easily interpret the relationship (one to many, many to many, zero or one, etc.).
I'm not looking for a full blown modeling tool that can generate SQL, more just a quick and easy diagram tool with appropriate ERD connectors.
Here's a sample image so you know what I'm talking about:
I can use a Windows, Linux or web based tool.