Popovici claimed victory in the men's 100m freestyle and McIntosh won the women's 200m butterfly.
In the other individual finals, Frenchman Leon Marchand completed the men's medley double in the 200m and Kylie Masse won a second Canadian double gold when she took the 50m women's backstroke.
In the women's 200m relay that ended the evening, the United States won gold, Australia took silver and Canada, with McIntosh collecting a second medal of the day, held on for bronze.
The evening had opened with a thunderclap when the US team announced that seven-time Olympic gold medallist Dressel, who had already scratched from the 100m free final, was quitting the championships for an unspecified medical reason.
Popovici, who had become the first Romanian man to win a world title when he won the 200m free, collected the 100m with a late surge.
"I'm a bit prouder of the 200m win, still this was great too," he said.
"Now I get to rest a little even though standing on my feet," he added.
"I feel good, I'm glad to have got two golds now, I think it will be pretty heavy carrying them."
McIntosh, a Canadian, broke the world junior record as she added the 200m butterfly to the silver she had won in the 400m freestyle on the opening day.
"It's one of my biggest dreams in the swimming world to be world champion and especially to do it in 200 fly...it is one of my favourite events," she said.
Popovici started the last lap trailing Canadian Joshua Liendo but accelerated past and then out-battled Frenchman Maxime Grousset to the line.
Popovici beat Grousset by 0.6sec and Liendo by 0.13.
Olympic champion Dressel had qualified for the final in a time slower than Popovici.
Popovici was asked if he had scared Dressel off.
"I don't think so, I think he is too big of a boy to be running away from someone like myself or frankly anyone but I hope he's ok and I hope he'll come back stronger."
Popovici's time of 47.58 was shy of the world junior record he set in the heats.
"This one hurt a little more, I guess I was more tired, more nervous I suppose, but I'm glad it's over and I get to have a rest."
McIntosh beat Hali Flickinger of the United States by 0.88sec with Zhang Yufei of China third.
"I just literally gave it my all and did everything I could, and put in all my energy and all my focus, and just stretched for the wall and put my hand on the wall as fast as I possibly could," McIntosh said.
Marchand, who claimed the 400m title on Saturday, grabbed the lead in the breaststroke and held it on the last lap to finish in 1min 55.22sec, edging out American Carson Foster and Japanese bronze medallist Daiya Seto.
"That was amazing," he said. "It was exciting to race these guys but it was pretty too. I saw everyone was going so fast over the first three legs that I had to do a big sprint in the last 50 and that was really painful."
The second Canadian gold medallist, Masse, a relative old timer at 26, had won gold in the 100m backstroke at the previous two world championships and a silver in Budapest. She has never won a major medal at the shortest distance.
"They really are just a flash, you don't really know where you are, especially on the backstroke, sometimes it doesn't necessarily feel great as you are kind of just going, but it's over before you know it, and you just have to put it all on the line," she said.
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