I absolutely love the first time I use an ability the DM doesn't know I have. The expression of "oh my god, you do what?" on his face is a pure delight.
-blarg
Shortly after 3E came out my DM had a Ring of Counterspell in the treasure. No one at the table was much impressed with it, so I selected it. I loved the first time I used it with Dispell Magic in it.
Illusion magic is my favouritest magic ever. It seems the most fundamentally magical of all, and yes, also provides the greatest potential for ingenuity and playful mischief and mayhem. Come Level 3 (or 4), Minor Image! Yesss...
I had a Changeling character stand in front of a tipsy dwarf and slowly shift her eyes to different regions of her face. One skill check later, the dwarf swore off drinking. Forever. She's semi-retired now, presumably corrupting random dwarves in Cormyr.
I once paid an artisan to create an exact replica of a miscellaneous magic item that nobody in the party wanted, enchanted it with Nystuls Magic Aura and then swapped it for the real thing. This was back in the day when you got xp for magic items. I think it was a folding boat but I am no longer positive.
Oh yeah, back on topic. Once running a campaign the Wizard player suddenly cast an anti-golem spell (something from the Spell Compendium which was allowed). The convenient part of it was that the spell magically appeared in his memory once the party figured out it was a clockwork fortress.
I once had a doppleganger character disguise himself as a character's dead mother and wake him up during the middle of the night over and over, telling him that he was in emminent danger then telling him to watch out for signs that the doppleganger would then cue the next day.