The possible existence of Micro Black Holes (and Mini Black Holes) was first suggested by Stephen Hawking in his 1971 paper 'Gravitationally collapsed objects of very low mass' in the Monthly Notices of the Royal Astronomical Society, Vol. 152, p. 75.
Hawking suggested that such holes were entirely compatible with Einstein's relativity theories, and that the smallest ones would have very little influence on ordinary matter - likely to pass through an object with the mass of Earth without interaction.
Current physics concepts suggest that the lower limit for the mass of a Black Hole is about 22 micrograms. According to Stephen Hawking's theories, however, such an entity would be inherently unstable, and would 'evaporate' almost immediately after its creation. Larger versions might be able to eventually accrue matter, but over a very long timescale (typically 10s of millions of years) The strength of their gravitational attraction following the same rules as any other object, i.e. directly proportional to its mass - which might be extremely low.
They have never been detected in any experiment to date.
In summary, it's currently not known if Micro Black Holes (or their remnants from 'evaporation') are common in the universe - or if they exist in Nature at all (for any appreciable length of time).
Note: Micro Black Holes gained attention recently with speculations that the Large Hadron Collider at CERN may now be sufficiently powerful to create them as a side effect of some collisions. 1). However, as mentioned above, Hawking theory proposes that they would instantaneously self-destruct. It has also been pointed out that cosmic rays, which permanently bombard almost all regions of the universe at all times, can also have the energies necessary to create Micro Black Holes. Further details : The case for mini black holes in the CERN Courier, Nov. 2004.