• new MTB frames now almost exclusively have a tapered headtube and take a fork with a tapered steerer;
• the fork steerer tapers from 1.5" to 1 1/8" (standard straight fork steerers are 1 1/8");
• many newer forks are only being made with tapered steerers (eg. new Rockshox Pike, SID 120mm world cups, etc.)

Some possible solutions

44mm head tube frames

If your frame has a 44mm headtube with semi-integrated headset then you can use a tapered steerer by swapping the lower bearing cup for an external 1.5" bottom cup. According to the Standardized Headset Identification System (S.H.I.S.), you want to find a bottom headset cup with the designation "EC44/40". This translates to External Cup to fit a 44mm headtube with a 40mm (1.5" bore). Several companies make such a bottom cup:

• Cane Creek 40-Series Traditional Conversion Bottom for 44mm Head Tube, 1.5' Crown Race Diameter
  
• Hope EC44/40;
• ACROS Headset Lower Part AH-44 S.H.I.S. EC44/40. Lower stack height 12.8mm - see technical data;
• Nukeproof Warhead EC44/40;
• Superstar components conversion headset