I have two little red serrated paring knives from Wüsthof in my life: one lives on my magnetic knife strip at home, and one is hidden in the drawer of my work station here in the Test Kitchen. I hide it because if someone else borrows it and I don't know where it is, I get very cranky. This is a knife I always want to have within reach.
What's so great about this little $9 knife? It's the best tool for thinly slicing rounds of citrus, and it's perfect for slicing tomatoes of any size. Ever tried cutting delicate slices of lemon with a regular paring knife, or slicing grape tomatoes in half with a bread knife? The sawing motion of the serrated knife makes it easier to slice through the tough skin and delicate flesh of citrus fruit without squishing or deforming it. And using a large bread knife to cut small ingredients (especially round ones, like tiny tomatoes) is a recipe for successfully slicing your fingertips off. (A good chef's knife can be used for both tasks, of course, but only if it's super-sharp, which is often not the case.)
The next time you're making tomato salad, use a serrated paring knife instead of your dull chef's knife, and the job will be so much easier. Need super-thin, gorgeous orange slices for an upside-down cake? Your small serrated knife is the right tool for the job. And speaking of cake, a serrated paring knife is perfect for trimming cake edges (or slicing off a bit to nibble on). It's also my favorite tool for evenly slicing chilled dough for any kind of slice-and-bake cookies.
You can buy a fancy mini serrated knife, but it's not going to perform any better than my $9 one, so don't bother. But if your regular paring knife needs to be replaced (and it probably does), you can also buy a set that also includes a serrated paring knife, like this one from Kuhn Rikon for $18 or this adorable ombre collection from Opinel for $25. Or get a whole set of serrated paring knives to use as steak knives at the dinner table as well as for prep work. Once you start using them, you'll want to have one stashed away in every drawer too.