Which open source GIS software to use for utility management?

As far as I know, there is not a full-fledged package for utility management in the open source domain. This is party because utility management has many facets such as material administration (closely tied with ERP systems), their role in the topological network and their behavior in specific circumstances.

You can find bits and pieces for each of these facets in the open source world (ex: for network topology, you can use PostGIS) however you need to wrap up and tie the blocks manually.

There are commercial QGIS extensions/addons for use cases such as water and electricity utility management, e.g. by German developers norBIT who are also heavily involved in QGIS development.

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.