Several nice ones posted so far. I particularly like the
Time Stop +
Prismatic Sphere appropriately positioned +
Reverse Gravity = inside-out enemies one.
I've seen plenty of combos in my own games, but the one that I'll mention first is a particular choice of
Shapechange- my Epic party's Sorceress was particularly fond of Extended-
Shapechanging into Beholder form and using her now-effectively-infinite
Disintegrates to blast convenient holes in enemy fortresses, dungeons, etc.
The spellcasters have also used that portal-
Gate trick, though in their case they did it just by using two
Gates- Have spellcaster A open one to flee through, then (on the other side) spellcaster B opens another directly in front of the first one leading to some unpleasant place. The last time they used this was in the Abyss, being chased by demons, so they opened the second one onto the ocean of Holy Water at the base of Mount Celestia- the party went to the Outlands for various secondary reasons of their own. One time they even did this in
Time Stop mode, since the party spellcasters all have Spell Stowaway (
Time Stop).
Time Stop cascades are brutal.
One tactic the party tanks have discovered and abused (via magic items obviously) is
Wraithstrike + full Power Attack + Dire Charge +
Haste and when possible, various other activated weapon powers like Fierce or Corrosive Surge to get extra damage. With all attacks that round being touch attacks, the damage potential of a Full Attack becomes truly staggering/sick. This, as part of a Scry/Buff/Teleport routine, often ends fights before they begin.
Oddly enough, my Epic party doesn't have any Rogues in it, so I haven't had to deal with Sneak Attack abuse myself. Of course, the abuses I do deal with are plenty.
EDIT: Oh, and how could I forget the perennial favorite of the second group?
Temporal Acceleration or
Time Stop + Barred
Forcecage +
Dimension Lock = enemy cooked to order. This one was used liberally in JollyDoc's Age of Wyrms campaign too (as posted in the Story Hour forum).