If it's just ArcMap that you are using then you could try QGIS, for the basic drawing of pipes and lines etc it's fine, I used to swap between ArcFM at work and QGIS at home for basic drawing, importing and exporting to Mapinfo Tab files and selecting, disolving and buffering the whole network, you did need to swap everything to a personal DB file (mdb) but that may have changed now that file databases have become an open standard(?), but the fancy ArcFM stuff, it doesn't do. I like Balkan's idea of using a PostGIS backend but haven't tried that, so can't really comment.
It really depends on what your needs are?
if you have some developers and DB administrators who are prepared to put time into setting up a postgis database
and do a cost benefit analysis if it's worth spending the time and development cost compared to the cost of commercial software.