The hardest part of UV unwrapping for me when I do 3D is that I have a hard time telling if the UVs are straight. I've tried to address that with this grid, which has both a square and a circle to give you multiple comparison points to see if the borders are straight. The rainbow gives you a largescale idea of how your image is tiled, and the numbering is a great help to see where your edges are and how much of the tiled texture they cut off.
This is just one I whipped together, here's a few others:Square Rainbow GridSquare Rainbow with Co-ordinate NumberingSquare Rainbow with Half InvertedBasic B/W NumberedMultiple Toned GridsMultiple Rainbow Grids
Hope someone finds this helpful
Note: I've put this up with a creative commons licence, so you are free to use it for commercial and personal use (but not to sell as is), and practically do whatever you like with it (it's just a grid after all right?) I would however like it if you did 2 things for me:
1) If you download and provide this somewhere else for reference, please link back to this page
2)If you use it as a base and make changes to the design, I'd love to see what you did and why you think it's better. I am always happy to learn of other things that would improve it.