The two commonly-used units for measuring atmospheric pressure are millibars and inches mercury. My Davis Vantage Pro2 weather station reports pressure in inches mercury and millibars are the typical measurement used for recording hurricane pressure. A normal millibar reading is 1000; an extreme measurement is 925. A normal inches mercury measurement is 30.0; an extreme measurement is 27.3. The Wikipedia pressure page provides a relatively easy conversion path via torrs. One millimeter mercury is equal to one torr, so one inch equals 25.4 torrs. One bar is equal to 750.06 torrs, so one millibar is equal to 0.75006 torrs.
Conversion from inches mercury to millibars simply amounts to conversion to millimeters/torrs, then conversion to millibars. The first involves multiplication by 25.4; the second involves division by 0.75006. The net scaling factor is 33.864. Conversion from millibars to inches mercury is simply the reverse calculation, so it simply involves multiplication by 0.02953.
So, 1000 millibars is equal to appx. 29.53 inches mercury. 30 inches mercury is equal to appx. 1016 millibars. A half-inch of mercury is equal to appx. 17 millibars. Here's a quick lookup table:
Update (10/26/10): It's useful to note that a hecto-Pascal (102 Pa) is equal to one millibar (10-3 bar). Equivalently, one bar is equal to 100,000 Pascals (105 Pa).