Here's a link to a seamless, circular washcloth I posted at Ravelry in 2010. I wrote this pattern to get rid of the usual sewn seam. The grafted "seam" creates a piece of knitting with no apparent beginning or end.
It takes less than 3 hours to complete and the Hexagonal version can be used in a pieced afghan when worked in worsted weight yarn.
The grafting may be a bit challenging for some, but with practice, you'll get it and even a seasoned knitter will be looking closely for the join.
You may have to log in first at Ravelry and then copy and paste the link. The pattern download link is on this page: