How to link to external files with relative path in QGIS actions?

You could try the eVis plugin for QGIS

http://biodiversityinformatics.amnh.org/open_source/evis/documentation.php#d0e390

Thanks to Rayner's answer, I can do what I want now - with some things to remember:

1. The relative path stored as attribute in data table should use forward slash (/), like: '/subfolder/file.ext' (without apostrophes) because my Linux Mint system won't work with back slash (\) while Windows XP can work with both styles.
2. The base path specified in eVis' Options tab should use your operating system's convention, that is: 'C:\path\to\project\folder\' (without apostrophes) on Windows and '/home/user/path/' (without apostrophes) on Linux.

Otherwise I will receive an alert that the file could not be found.