Similar to Adobe Illustrators 2d curve blend, this tool takes a series of curves and creates interpolated steps between them.

Rhino Script

Option Explicit 
'Script written by <David Mans> 
'Script copyrighted by <NeoArchaic Design> 
'Script version Sunday, May 03, 2009 1:54:38 PM 
Call Main() 
Sub Main() 
	Dim Curves, SM, DN 
	Curves = Rhino.GetObjects("Select Curves", 4) 
	If isNull(Curves) Then Exit Sub 
	SM = Rhino.GetBoolean("Blend Type", array("Type", "Straight", "Smooth"), array(False)) 
	If isNull(SM) Then Exit Sub 
	DN = Rhino.GetReal("Blend Density", 5, 1) 
	If isNull(SM) Then Exit Sub 
	Call Rhino.EnableRedraw(False) 
	If uBound(Curves) > 0 Then 
		Call BlendCurves(Curves, SM(0), DN) 
	Else 
		Call Rhino.Print("2 or more curves required") 
	End If 
	Call Rhino.EnableRedraw(True) 
End Sub 

Function BlendCurves(arrCurves, blnSmooth, dblDensity) 
	BlendCurves = Null 
	Dim i,j,p 
	Dim count, max, dMax 
	Dim arrptCount(), arrCrvDeg(), arrPointSet(),arrPts(), arrCrvDom(),arrCrvStep(),arrBlndCrvPt() 
	Dim arrBlendCurve(), arrCtrlCrv() 
	Dim dblSmoothness 
	If 	blnSmooth = False Then 
		dblSmoothness = 1 
	Else 
		dblSmoothness = 3 
	End If 
	count = Ubound(arrCurves) 
	ReDim arrptCount(count), arrCrvDeg(count), arrCrvDom(count), arrPointSet(count), arrPts(count) 
	'get control point count 
	For i = 0 To count Step 1 
		arrptCount(i) = Rhino.CurvePointCount(arrCurves(i)) 
		arrCrvDeg(i) = Rhino.CurveDegree(arrCurves(i)) 
	Next 
	'determine max point count 
	max = Rhino.Max(arrptCount) 
	dMax = Rhino.Max(arrCrvDeg) 
	'rebuild curves with max control point count 
	For i = 0 To count Step 1 
		Call Rhino.RebuildCurve(arrCurves(i), dMax, max) arrPointSet(i) = Rhino.CurvePoints(arrCurves(i)) 
	Next 
	max = max - 1 
	ReDim arrCtrlCrv(max), arrCrvDom(max), arrCrvStep(max), arrBlndCrvPt(max) 
	'create control curves 
	For i = 0 To max Step 1 
		For j = 0 To count Step 1 
			arrPts(j) = arrPointSet(j)(i) 
		Next 
		arrCtrlCrv(i) = Rhino.AddInterpCurve(arrPts, dblSmoothness) 
		arrCrvDom(i) = Rhino.CurveDomain(arrCtrlCrv(i)) 
		arrCrvStep(i) = (arrCrvDom(i)(1) - arrCrvDom(i)(0)) / (count * dblDensity) 
	Next 
	'create blended curves 
	ReDim arrBlendCurve(count*dblDensity) 
	For i = 0 To count * dblDensity Step 1 
		For j = 0 To max Step 1 
			arrBlndCrvPt(j) = Rhino.EvaluateCurve(arrCtrlCrv(j), arrCrvDom(j)(0) + i * arrCrvStep(j)) 
		Next 
		arrBlendCurve(i) = Rhino.addcurve(arrBlndCrvPt, dMax) 
	Next 
	'delete control curves 
	Call Rhino.DeleteObjects(arrCtrlCrv) 
	Call Rhino.DeleteObjects(arrCurves) BlendCurves = array(arrBlendCurve, arrCtrlCrv) 
End Function