Subclass of NGUI's UIGrid that adds support for variable size cells. Update: Turns out it's mostly unnecessary; UITable does much of the same - DOH

UIVariableCellSizeGrid.cs

using UnityEngine;
using System.Collections.Generic;

// This is a subclass of NGUI's UIGrid that supports cells of varying sizes,
// in addition to fixed sizes.
// http://ghostcrabworkshop.com

public class UIVariableCellSizeGrid : UIGrid
{
	// If neither of these is set to true, spacing will be exactly as it 
	// is in UIGrid.
	public bool useCellCollidersForHeight = false;
	public bool useCellCollidersForWidth = false;
		
	// Set this delegate to sort the cells in a different way.
	// Should return 0 if both should be at the same rank.
	// Should return a positive value if a ranks higher than b.
	// Should return a negative value if b ranks higher than a.	
	public delegate int SortDelegate(Transform a, Transform b);	
	public SortDelegate customSortMethod = null;
	
	protected int sortMethod(Transform a, Transform b) 
	{ 
		if (this.customSortMethod != null)
		{
			return this.customSortMethod(a, b);
		}
		else
		{
			return UIGrid.SortByName(a, b);
		}
	}
	
	public override void Reposition()
	{
		if (!mStarted)
		{
			repositionNow = true;
			return;
		}

		Transform myTrans = transform;

		int x = 0;
		int y = 0;
		
		int rightEdgeOfPrevVariableCell = 0;
		int bottomEdgeOfPrevVariableCell = 0;

		if (sorted)
		{
			List<Transform> list = new List<Transform>();

			for (int i = 0; i < myTrans.childCount; ++i)
			{
				Transform t = myTrans.GetChild(i);
				if (t) list.Add(t);
			}
			list.Sort(this.sortMethod);

			for (int i = 0, imax = list.Count; i < imax; ++i)
			{
				Transform t = list[i];
				
				this.updateCellTransformPosition(t, x, y, 
					ref rightEdgeOfPrevVariableCell, 
					ref bottomEdgeOfPrevVariableCell);
				
				if (++x >= maxPerLine && maxPerLine > 0)
				{
					x = 0;
					++y;
				}
			}
		}
		else
		{
			for (int i = 0; i < myTrans.childCount; ++i)
			{
				Transform t = myTrans.GetChild(i);

				this.updateCellTransformPosition(t, x, y, 
					ref rightEdgeOfPrevVariableCell, 
					ref bottomEdgeOfPrevVariableCell);

				if (++x >= maxPerLine && maxPerLine > 0)
				{
					x = 0;
					++y;
				}
			}
		}

		UIDraggablePanel drag = NGUITools.FindInParents<UIDraggablePanel>(gameObject);
		if (drag != null) drag.UpdateScrollbars(true);		
	}
	
	// Updates the position of the given transform and updates the 
	// rightEdgeOfPrevVariableCell and bottomEdgeOfPrevVariableCell params, if we're in 
	// variable cell size mode.
	protected void updateCellTransformPosition(Transform t, int col, int row,
		ref int rightOfLastCell, ref int bottomOfLastCell)
	{
		if (!NGUITools.GetActive(t.gameObject) && hideInactive) return;

		float depth = t.localPosition.z;
		int cellX = 0;
		int cellY = 0;
		if (arrangement == Arrangement.Horizontal)
		{					
			if (this.useCellCollidersForWidth)
			{
				Vector3 cellSize = 
					UIVariableCellSizeGrid.colliderSizeForTransform(t);

				// The current cell's center should go at the right edge of the
				// last cell plus half the width of the current cell.
				int cellHalfWidth = (int)(cellSize.x/2);
				cellX = rightOfLastCell + cellHalfWidth;
				
				// Record the right edge of this cell for the next iteration's 
				// use.
				rightOfLastCell = cellX + cellHalfWidth;
			}
			else
			{
				cellX = (int)(cellWidth * col);
			}
			
			cellY = (int)(-cellHeight * row);
		}
		else // Arrangement.Vertical
		{
			cellX = (int)(cellWidth * row);
					
			if (this.useCellCollidersForHeight)
			{
				Vector3 cellSize = 
					UIVariableCellSizeGrid.colliderSizeForTransform(t);
				
				// The current cell's center should go at the bottom edge of the
				// last cell plus half the height of the current cell.
				int cellHalfHeight = (int)(cellSize.y/2);
				cellY = bottomOfLastCell - cellHalfHeight;
				
				// Record the right edge of this cell for the next
				// iteration's use.
				bottomOfLastCell = cellY - cellHalfHeight;
			}
			else
			{
				cellY = (int)(cellWidth * row);
			}
		}
		// Using ints even though Vector3 takes floats to avoid half-pixel 
		// values.
		t.localPosition = new Vector3((float)cellX, (float)cellY, depth);
	}
	
	static protected Vector3 colliderSizeForTransform(Transform transform)
	{
		Collider collider = transform.gameObject.GetComponent<Collider>();
		if (collider)
		{
			return collider.bounds.size;
		}
		else
		{
			return Vector3.zero;
		}
	}
}