Dxtromethorphan, or DXM, is a dissociative NMDA-antagonist found in over-the-counter cough medicines, such as Robitussin Max Strength for Dry Coughs. Many people assume the DXM Angel is named for this drug, but the official line is that DXM in this case stands for 'Do Xercise More'
In South Park, Butters takes DXM and reports, "Fellas - I'm feeling kinda bowling-ballish." That's a pretty good description of what it's like.
- Erowid DXM vault
- William White's DXM FAQ - Very possibly the best and most comprehensive text ever written about any subject.