I've bought a lot of stuff over the years. Some of it just sits there, collecting dust... some of it I can't even remember the point. Thankfully, every now and then, something comes along that you end up using again, and again and again.
For me, the perfect tool is Karen Kay Buckley's Perfect Circles for applique. These little jewels are thin, durable, heat-resistant plastic that allow you to gather fabric around the edge, starch it and then push the iron over the edge for a perfect circle. Yep, it's that easy.
There are 16 sizes in the package, 4 of each size. Karen also has a set for larger circles.
If you are doing edge-turn applique, these are a must have! You'll wonder how you ever made circles before. Here's a link to Karen's site. She teaches also and her class is excellent.