A runestaff (MIC p. 223) allows a character who's attuned to it to sacrifice one of their spell slots to cast a spell of equal or lower level from the staff. This allows characters to cast spells that they don't know.
However, the spell must be on the user's class spell list. From the item description, MIC p. 224:
By expending a prepared arcane spell or arcane spell slot, the wielder can cast a spell of the same level or lower from the runestaff’s list, as long as that spell also appears on the wielder’s class spell list.
If a character has more than one arcane spellcasting class (say, a Duskblade/Wizard) can they expend a spell slot from one class (say, a Duskblade slot) to cast a spell that appears exclusively on the list of their other class (say, Wraithstrike)?