WoW:API UnitLevel

Retrieves the unit's level. unit being target,party,pet etc.

EX:

 /script SendChatMessage("Level "..UnitLevel("target"));

will say, if you're targeting a lvl50: Level 50

As of some patch (1.5?) it will return -1 if the level is unknown (ie, you're too low to know it, or it's a special boss)
--- tarp (first post, probably broke all the design rules :) )

(sorry for the bad format and all, but wanted to get something up)