SHAPEFILE PROJECTIONFINDER is an approach in context with gis data and unknown projections. The target group of this program are mainly user who wants to integrate gis data without great effort in their projects and have no spezialised knowledge about coordinate systems.
You can see a video of the presentation in Salzburg and the following discussion in German HERE. With a click into the graphic (right) you get the FOSS4G poster presentation.
You can download a free version of this tool using registered EPSG-Codes of geotools 2.6.0.
IMPORTANT: It is possible to generate WKT (Well-Known-Text) for .prj-files using OGC or ESRI definitions .
Using Web Feature Services (WFS), like these of Bundesamt für Kartographie und Geodäsie, similar webservices comparable to www.projfinder.com would be possible for administrative units. You could project extents of administrative units in geographic coordinates in different projections and if your unknown data intersects one or more of this projected extents, you get a small list of possible .prj for your shapefile.
The basic idea to use administrative units was developed by The Austrian Service for Torrent and Avalanche Control.