I love it when I'm watching other shows and I see DOOL people in them. Bo and Lucas are the two I've seen the most.
I've also seen Maxine, Melanie, Steve, and Chloe. I'm sure there are others but I can't think of them at the moment.
I see a LOT of crossover is between DOOL and General Hospital. Watching Friday's episode right now, Eve Donovan just showed up. Harris Michaels on Days used to play Jason on GH. Shane Donovan is also Victor Casadine on GH.