Well, I can think of quite a few songs about SI, but... if I try to think of slow songs, the list suddenly becomes much shorter...
What I can think of right now is:
Sinergy-Razorblade Salvation
Linkin Park-Braking the habit (I'm not sure if this one was mentioned before...)
Children of Bodom-Everytime I die (not excactly about SI, but I think it still kind of fits. Also not really slow (at least not what an average person would call slow))
Children of Bodom-Touch like angel of death (again, not really slow but has some great verses about SI'ing)
CoB-Towards dead end (see above)
For my pain-My wound is deeper than yours
For my pain-Tomorrow is closed gate (more about being suicidal than SI, but still catches the mood well.)
Sonata Arctica-Victoria's secret
Sonata Arctica-Replica (again, not really about SI but feels like it is)
Papa Roach-Last Resort (no slow, but I guess one of the most "popular" songs about SI)
If anyone's interessted, I can also post a list of German songs which have SI as a topic
