I was looking for a way to link an attribute table of a vector layer to a look up table, with a one to many relation. As discussed here and here, this does not seem to be possible in QGIS. One of the suggestions was to use the “eVis Event Browser”. This plugin allows you to link to external documents. Although a neat plugin, it is not the same as creating an one-to-many link to a lookup table.
There is another opensource GIS, gvSIG that does offer this option. You can link any table to an attribute table of a vector layer by using the ‘Link’ function (see arrow in screenshot below). After establishing the link clicking on a feature in your vector layer will highlight all the linked records in the lookup table. See the screenshot for an example.