How to make an isometric grid in Inkscape 0.48.1, and have your drawings snap to the grid points.
If you haven't heard, Inkscape is a free, open source vector graphics drawing program, whose native format is the SVG, the web standard. Go download it, and read on.
Inkscape has a built-in square grid, but other grids must be done manually. Here is how to do the 2-dimensional triangular lattice (or the isometric grid, or the triangles grid, or triangle graph paper), and have it work just like the built-in grid, where you can draw over it, and have your work snap to the grid points. I'm using the latest version, Inkscape 0.48.1 on the Mac.
Use the star/polygon tool to make a hexagon. Hold down the control key to help you line up a flat edge with the horizontal.
Open the "Fill and Stroke" dialogue (Shift+control+F), select no fill, a light colour stroke paint, and a thin-ish Stroke style.
Select "Snap to cusp nodes" and "Snap from and to midpoints of line segments", in the Snap Controls Bar (you can show/hide this bar in the view menu).
Use the "Draw Bezier curves and straight lines" tool to draw a diagonal into the hexagon. It should show you that it is snapping to the cusp nodes. Now draw a line from the other corners to the midpoint of that diagonal. Fix the fill and stroke by copying from the hexagon's fill and stroke settings when you are done.
Group your hexagons and lines with control+G.
Open the Create Tiled Clones dialogue (edit->clone), and select the "Shift" tab. Set Rows,Columns to small numbers for experimenting (500x500 could take your computer all day to create). Set "Shift X" to -25% per column, and "Shift Y" to 50% per column, just like in the picture. Press create. Control+Z to undo. When satisfied, increase the rows and columns and make your grid as large as you need it.
Now open the layers dialogue (Shift+control+l), and name the grid layer something useful, like grid, lock it, and make a new layer for your drawing. You may want to shut off "snapping to midpoints" now, but leave cusp node snapping on.
Now go crazy!
Here is a drawing I made by dragging clones of tiles to the grid, and snapping them into place.