He discovered lysergic acid diethylamide.
From the Los Angels Times' obit:
On Friday afternoon, April 16, 1943, Hofmann had just completed synthesizing a new batch when, he subsequently wrote his supervisor, "I was forced to interrupt my work in the laboratory in the middle of the afternoon and proceed home, being affected by a remarkable restlessness, combined with slight dizziness.
"At home, I lay down and sank into a not-unpleasant intoxicated-like condition, characterized by an extremely stimulated imagination. In a dreamlike state I perceived an uninterrupted stream of fantastic pictures, extraordinary shapes with intense, kaleidoscopic play of colors. After some two hours, this condition faded away."
Hofmann suspected that the state had been caused by something in the lab. In an interview on his 100th birthday, he said, "I didn't know what caused it, but I knew that it was important." ...
The following Monday, he took what he considered to be an extremely small dose of LSD, so small that a similar dose of even the most powerful toxin known at the time would have had little or no effect. He had planned to gradually increase the dosage, but instead was surprised to encounter the first bad acid trip.
Feeling bad, he asked his laboratory assistant to accompany him home on his bicycle, no cars being available because of wartime restrictions. During the trip, "I had the feeling that I could not move from the spot. I was cycling, cycling, but the time seemed to stand still." ...
Hofmann thought he was dying and sent for a doctor, but the physician could find nothing wrong.
After about six hours, the experience began to change into a pleasant one. "After some time, with my eyes closed, I began to enjoy this wonderful play of colors and forms, which it really was a pleasure to observe. Then I went to sleep and the next day I was fine. I felt quite fresh, like a newborn."