I

iconTextButton
iconTextCheckBox
iconTextRadioButton
iconTextRadioCollection
iconTextScrollList
iconTextStaticLabel
ikfkDisplayMethod
ikHandle
ikHandleCtx
ikHandleDisplayScale
ikSolver
ikSplineHandleCtx
ikSystem
ikSystemInfo
illustratorCurves
image
imfPlugins
inheritTransform
insertJoint
insertJointCtx
insertKeyCtx
insertKnotCurve
insertKnotSurface
instance
instanceable
instancer
internalVar
intersect
interToUI 
intField
intFieldGrp
intScrollBar
intSlider
intSliderGrp
iprEngine
isAnimCurve 
isConnected
isDirty
isolateSelect
isParentOf 
isSameObject 
isTrue
isValidObjectName 
isValidString 
isValidUiName 
itemFilter
itemFilterAttr
itemFilterRender
itemFilterType

indicates a MEL script