They walked down the main street past Zonko's Joke Shop, where they were unsurprised to see Fred, George, and Lee Jordon... (The Order of the Phoenix, 335)Fred and George began making joke products as a mail order service. After Harry gave them his Tri-Wizard earnings though, they had much more resources to develop their products and began testing them out at Hogwarts. Notably, they create the Ton-Tongue Toffee and skiving snackboxes, full of sweets that made a student ill enough to skip class.
When they opened their shop in Diagon Alley, it was wildly successful, despite the dark times:
Set against the dull, poster-muffled shop fronts around them, Fred and George's windows hit the eye like a firework display. Casual passersby were looking back over their shoulders at the windows, and a few rather stunned-looking people had actually come to a halt, transfixed. The left-hand window was dazzlingly full of an assortment of goods that revolved, popped, flashed, bounced, and shrieked; Harry's eyes began to water just looking at it. (The Half-Blood Prince, 116)