A very simple but good looking when they are all put together signature/friendship/album block is The X-Quisite block. You can put several of them together to create a secondary pattern of floating stars or X's & O's.
http://www.fvqg.org/01archiv/0201.htm The link is for a single 6" block. Normally, when you purposely set out to make a secondary design, you'd make the single block much smaller but for a signature type block, 6" is a good size.