I have taken over updating this addon since TMcMahon51 is no longer playing WoW and he gave me permission to continue with it. It is working (hopefully) in patch 4.2. I have done nothing more than fix a few errors and bump the .toc. I believe I have fixed (credit goes to suicidalkatt) the units bug. Please try it out and let me know if you have any issues.
If nightcracker or TMcMahon51 return to this addon I will delete this.
Description taken from the original addon at http://www.wowinterface.com/downloads/info15077-ncSpellalert.html
ncSpellalert is an addon for alerting you when an enemy casts an interuptable or important spell, your enemy gains an important buff or an enemy drinks/trinkets. This addon is based on evl_Spellalert so a lot of credit for evl. Please give feedback and tell me if I need to add/remove certain spells to the proc list.
Download & go! To change/add/delete spells that it should notify, open config.lua with your favorite plain text editor and start editing! By default this addon is only enabled in PvP zones, to show it in all zones set "pvponly" to false in config.lua. Also, by default this addon messages for all units when they cast important spells or get important buffs, but you can also make it alert for only a few units by setting "allunits" to false in config.lua and changing "units" to your wishes(defaults to target and focus only).