Common Postranslational Modifications

Sulphydryls
(C)		 Disulphide bond-2.0159 Oxidation+15.9994
Cysteinylation+119.1442 Glutathionylation+305.3117

Amines
(K/N-terminus)		 Methylation+14.0269 Formylation+28.0104
Acetylation+42.0373 Lipoic acid+188.3147
Farnesylation+204.3556 Myristoylation+210.3598
Biotinylation+226.2994 Palmitoylation+238.4136
Stearoylation+266.4674 Geranylgeranylation+272.4741

Acids & amides
(E/D/Q/N)		 Pyroglutamic acid (Q)-17.0306 Deamidation (Q/N)+0.9847
Carboxylation (E/D)+44.0098

Hydroxyl groups
(S/T/Y)		 Phosphorylation+79.9799 Sulphation+80.0642

Carbohydrates
(S/T/N)		 Pentoses+132.1161 Deoxyhexoses+146.1430
Hexosamines+161.1577 Hexoses+162.1424
N-acetylhexosamines+203.1950 Sialic acid+291.2579

If you don't see your modification here, try the modifications list from the University of Melbourne, Delta mass 2.0.