According to the work of A. Duquennoy and M. Mayor, 57% of systems
have two or more stars.  They were working with a sample of F and G
stars, i.e., stars like the Sun.  It appears that for the coolest,
low-luminosity stars (the M-dwarfs) there are fewer binaries.  Fischer
and Marcy found that only 42% of M-dwarfs are binaries.  Neill Reid
and I have used HST images to find that for the coolest stars in the
Hyades cluster (absolute magnitude > 12, or mass < 0.3 solar masses)
only 30% are binaries.

[There's also the tongue-in-cheek answer that three out of every two
stars is in a binary.  TJWL]

References:
Gizis, J. & Reid, I. Neill  1995, "Low-Mass Binaries in the Hyades,"
     Astronomical Journal, v. 110, p. 1248

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