One of the biggest obstacles anglers face to catching fish subsurface is detecting the bite. I am convinced that most anglers get more "bites" than they realize. During the winter months, when lakes and parts of rivers cap with the cold hard stuff, detecting the bite can become difficult. Fish feel protected under the ice, the cold water slows their metabolism, and they are in no great hurry to eat. The sluggishness increases as the ice grows thicker and the winter season wears on. That is why anglers, especially ice fishermen, have to take steps to recognize even the lightest of bites.
Spring bobbers have been around for a long time. They are basically some kind of indicator (bobber) attached to the end of a spring (or wire) that allows the angler to see even the softest of bites/movement. The thicker the wire/spring, the less sensitive the indicator, and vice versa. Basic spring bobbers don't cost too much in stores. You can often get a pack of two for 99 cents. The nicer ones can cost a few dollars for one bobber.
Here's a really brief tutorial showing how you can create your own. They won't save you millions of dollars, though they will save you some. They also allow you to make spring bobbers with various sensitivities, according to your needs.
Needle nose pliers, with a wire cutter.
Guitar strings (used/old is cheapest, usually free)
Corkies (Whatever color you feel is most visible) (I prefer a small size)
Masking tape (or whatever tape you prefer, to attach bobber to rod tip)
If you have any helpful tips or improvements, please share! Good luck, and be safe on the ice.