Physically Connecting and Tracing Ethernet Cables

We just recently remodeled out office.  Previously, the area we remodeled was one very large room which we've split into four offices.  This large room had multiple Ethernet outlets all which led to a data closet where the cable all come together and just hang with not end on them so that the individual wires are exposed.  There are probably 40 separate CAT5 cables that come together here.  Each of the new offices have 2 Ethernet outlets in them, one on either side of the office.  I need to somehow trace these down to their source in the data closet.  I know I can buy a tool of some kind to do this but didn't know if maybe there was some simple homemade tool or trick that I could utilize.