What is the proper way to run wires along/across basement ceiling joists?
I realize this is an old question, however I just wanted to mention something... Tester101 indicated not to drill too many holes within the joist, as it could weaken it and cause structural damage.  I thought I should clarify something... The principal force that is applied to a floor joist is moment.  With that, you get tensile along the bottom edge, and compression along the top edge.  The other force would be shear, however moment is your main one.  Being that it is primarily a moment force, the relative force implied to the joist would zero-out the closer you are to the centre (width).  In other words, the centre (width) of the joist is your pivot point.  That being said, as long as you drill in the centre, or as close to the centre as possible, you won't significantly reduce the joist's load-bearing capacity.  (Just think of a truss joist, or an I-joist... that will give you an idea of how little the force is in the centre).  So for a 2x10 joist, you would drill about 4.25" from the bottom or top, as that should be smack in the centre.  The amount of holes doesn't matter, as long as you don't touch the edges.  Of course, this is within reason, if you drill out 90% of the entire centre you would leave very little shear resistance.  Coincidentally, this is also why an I-beam is shaped that way.  There's relatively little load in the centre (the thin part), whereas most of the loading is tension along the bottom and compression along the top.  I should also note that if the joist is cantilevered, than the whole thing reverses on the cantilevered section (compress along bottom, tension along top).

As a structural engineer, I can tell you that the worst thing you can do is "notch" the joist.  By that I mean cutting out a section of the bottom or top edge of the joist.  If you cut out a section of the bottom edge, which becomes under tension when a moment is applied, the joist would be significantly weakened.  Similarly, if you cut out a section of the top edge, which would be under compression when a moment is applied, the joist would be significantly weakened.

Sorry for the long answer... it's a slow day.  But hopefully this knowledge comes to good use for someone.