Hi Michel,
Well, you could do this easily with an app. Assuming you've got a column in your node table that has the URL for the link then your app would listen for node selection and then open the appropriate page. That *may* not be what you want, though, since clicking on a node (node selection) is a common practice and *most* of the time users would not want to see a web page. An alternative might be to implement a NodeViewTaskFactory, which will allow you to respond to double-clicks.
-- scooter