When Danielbeast of lonelygirl15 is shown to be wearing "English flag underwear" (actually the Union Flag) in "Crazy Emo Chick", he protests that he hasn't had a chance to do laundry lately, so they were all he had to wear.
In Homestar Runner, somebody once sent in a photograph of their Stinkoman underwear.
When the Monkey King came to Whateley Academy in the Whateley Universe, he raided panties from every girls' dorm there. Some of the trophies included teddy bear panties, and a pair of My Little Pony panties that he left on the flagpole. Rumor has it they belong to Dragonrider, the daughter of supervillain Devilmaster.