Please reference this thread for a full explanation of the chassis numbers.

First, all M-B engines have a 6-digit model number, similar to the chassis number. The first three digits indicate what "basic" engine it is, with the last three digits indicating what "version" of the engine it is (i.e. is the engine turbocharged, does it have an EGR setup, etc). The model number on an M-B gas engine begins with the letter M ("motor", I think), while the model number on an M-B diesel engine begins with the letters OM ("oel motor"; basically, "oilburner ). I'm limiting this to diesel engines, since I know jack-squat about the gas engines...and I'm going to try and keep this chronological. Again, these are only engines as seen in U.S. models; European models ARE different (for instance, the OM616.912 was only sold in the U.S. until 1983, but was sold until 1985 in Europe). On that same note, keep in mind that there are Euro-market cars floating around in the U.S. that, due to having been originally sold in Europe, may not conform to this list. If you come across a car that does not match anything in this list, you are almost certainly looking at a Euro-market car. Furthermore, I opted to not include the new Common-Rail Direct-Injected engines (CDI's) that are available today, due to a lack of readily available information.

The following are cast-iron block, cast-iron head indirect-injected (IDI) engines. All are single overhead camshaft engines, with a timing chain connecting the crankshaft to the camshaft and to the injector pump drive gear. All have the OHC equivalent of solid lifters, requiring a valve adjustment performed every 15K miles. The OM61x engines listed below are the most common engines you will see in a U.S. M-B diesel, and did a lot to solidify the reputation M-B diesel engines have of lasting forever.
OM636.930 - 1.8l (I think) inline-4 n/a, used between 1953 and 1961. The OM636 and the OM621's are likely the same basic engine block, and are unique compared to other engines.
OM621.910 – 1.9l (I think) inline-4 n/a, used between 1958 and 1961.
OM621.914 - 1.8l (again, I think) inline-4 n/a, used between 1961 and 1962.
OM621.912 - 1.9l inline-4 n/a, 60hp, used between 1962 and 1965.
OM621.918 - 2.0l inline-4 n/a, 60hp, used between 1966 and 1967.
OM615.913 - 2.0l inline-4 n/a, 61hp, unknown age span. The OM615, OM616, and OM617 share a basic block design and construction (although there were many changes, including adding a cylinder in the OM617).
OM615.912 - 2.2l inline-4 n/a, 60hp, used between 1968 and 1973.
OM616.916 - 2.4l inline-4 n/a, 62hp, used between 1973 and 1976.
OM617.910 - 3.0l inline-5 n/a, 88hp, used between 1975 and 1976.
OM616.912 - 2.4l inline-4 n/a, 67hp, used between 1977 and 1983. Oil filter relocated as compared to 616.916, due to different chassis design.
OM617.912 - 3.0l inline-5 n/a, 88hp, used between 1977 and 1981. Oil filter relocated as compared to 617.910, due to different chassis design.
OM617.950 - 3.0l inline-5 turbocharged, 110hp, used between 1978 and 1980. Note that this is the only turbocharged 617 that did NOT have factory EGR equipment until mid 1980 production.
OM617.951 - 3.0l inline-5 turbocharged, 120hp, used between 1981 and 1985. EGR equipment added.
OM617.952 - 3.0l inline-5 turbocharged, 120hp, used between 1981 and 1985. Believed to be identical to 617.951, except for minor differences to accommodate the W123 chassis.

The following are cast-iron block, aluminum head indirect-injected (IDI) engines. Like the cast-iron engines, a timing chain connects a single overhead cam (except for the OM606, which has twin overhead cams and 4 valves per cylinder) and the injector pump drive gear to the crankshaft, but the OHC version of hydraulic lifters are used, eliminating the need for periodic valve adjustments. Some later engines have electronic controls for the EGR, and possibly for pump timing as well. All of the following engines share a basic block design and construction, although like the OM61x engines there were many changes. These engines share very little with prior designs.

OM601.921 - 2.2l inline-4 n/a, 72hp, used between 1984 and 1986.
OM603.961 - 3.0l inline-6 turbocharged, 148hp, used between 1986 and 1987.
OM603.960 - 3.0l inline-6 turbocharged, 148hp, used in 1987.
OM602.911 - 2.5l inline-5 n/a, 93hp, used between 1987 and 1989.
OM602.961 - 2.5l inline-5 turbocharged, 123hp, used in 1987.
OM602.962 - 2.5l inline-5 turbocharged, 123hp, used between 1990 and 1993. Believed to be identical to 602.961 except electronic controls for emissions reasons.
*OM603.970 - 3.5l inline-6 turbocharged, 134hp, used between 1990 and 1991.
*OM603.971 - 3.5l inline-6 turbocharged, 148hp, used between 1992 and 1994.
OM606.910 - 3.0l inline-6 n/a, 134hp, used in 1995
OM606.912 - 3.0l inline-6 n/a, 134hp, used in 1997. Believed to be identical to 606.910 except for minor differences to accommodate a different chassis.
OM606.? - 3.0l inline-6 turbocharged, 174hp, used between 1998 and 1999.

*WARNING FOR THIS ENGINE: There was a defect in the building of many of these 3.5l engines. As the rods begin to fail, they typically make the piston move about in the bore, ovaling out the cylinder to the point where the cylinders can no longer hold compression. For whatever reason, I don't believe simply re-sleeving the cylinders will correct this, so this can only be repaired by replacing the entire shortblock. I believe that some later engines were built with the correct rods after M-B realized that they had this problem, and many cars out there have already had the blocks replaced. But, I do not know of an external method of determining whether the engien has the old or the new connecting rods, so BEWARE when looking at any car powered by one of these engines. This problem is limited to the OM603.97x 3.5l engines.

Based on the sticky from www.oilburners.net and edited to update the information.