AEM makes this great cold air intake for the NC model Mazda MX5. Also sold by Mazda as the MazdaSpeed intake if you want to pay more for the exact same intake!
The stock MX5 air-box is restrictive by its design to allow use of an inexpensive and flat paper filter. The AEM intake achieves better breathing by using a much higher flowing velocity stack conical filter placed directly in the flow of cool air in the "mouth" of the MX5.
Not only will this intake give your MX5 much freer breathing and quicker acceleration, but also provide a nice deep growl at high revs. This is the same intake that MAZDA sells for all MX5s in the new Pro Spec racing series in the U.S.
Note that install on 2009 and beyond is commonly accomplished by using the upper factory elbow instead of the rubber elbow included with this kit (or you can do it the hard way and use the AEM elbow if buy a 90 degree PVC socket from 06-08 MX5, and Mazda bracket LFG1-18-835B).