If you’re looking for an alternative to Visio, check out Google Docs Drawing. Visio costs about £229 which if you’re only doing occasional diagrams seems a bit steep, while Google Docs is free.
Google Docs Drawing isn’t as powerful as Visio, so it may not suit all users, but for what I need – drawing diagrams showing the overviews of system designs, it does the job. I can draw shapes, link them and the links will move as shapes are moved. It also has the benefit of being a hosted document that I can share with the client. They, or I can modify it without having to manage versions.