Void Shift+Desperate Prayer is a pretty common one. Actually I don't have a lot that are disc-specific, since I have everything keybound, even rarely-used stuff.
One I do particularly like that's generic-
/cast [mod:alt,harm] Shackle Undead
/cast [mod:alt,help] Fear Ward
/castsequence [nomod,harm] reset=20 Void Tendrils, Psychic Scream
For real PVP I'd sometimes change the last one to Scream then Psyfiend/VT, but usually I like harassing enemy healers.
Another good set of combined abilities (stance:1 is shadowform)
/use [nomod,stance:1] Potion of the Jade Serpent
/cast [nomod,stance:1] Shadowfiend
/castsequence [nomod,stance:0] reset=55 Shadowfiend, Alchemist's Rejuvenation
/castsequence [mod:alt] reset=300 Hymn of Hope, Potion of Focus
and a *big* ugly-
/cast [mod:alt,harm][stance:1,harm,combat]Dispel Magic
/castsequence [nocombat][mod:alt,noharm]reset=9 Inner Fire, Power Word: Fortitude
/equipset [spec:2]Holy; [stance:1]Shadow; [spec:1]Disc
/use 0 1
(the /use 0 1 is my alchemist's flask, always in the same bag slot) Offensive dispel, buffs, friendly dispel and auto-equipping the right gear. Multipurposed the hell out of that button.
Edited by Breathkeeper on 2/28/2013 2:55 PM PST