NumberSeparator is an option for NumberForm and related functions which gives the string to insert at breaks between digits.
OpenAppend["file"] opens a file to append output to it, and returns an OutputStream object.
PlotRangePadding is an option for graphics functions that specifies how much further axes etc. should extend beyond the range of coordinates specified by PlotRange.
SectorOrigin is an option to PieChart and related functions that specifies where sectors should start.
SelfLoopStyle is an option for GraphPlot and related functions that specifies how to draw self-loops that connect a vertex to itself.
SignificanceLevel is an option to VarianceTest and similar functions that controls cutoffs for diagnostic tests as well as test conclusions.
SimilarityRules is an option for functions such as SequenceAlignment that gives a list of rules for similarity scores to assume between pairs of elements.
StatusArea[expr, string] displays string in the status area of the current notebook when the mouse pointer is in the region where expr appears.
VectorPoints is an option to VectorPlot, ListVectorPlot, and related functions that determines how many field vectors to draw.
VertexLabeling is an option for GraphPlot and related functions which specifies whether labeling should be included by default for vertices in graphs.