A couple of months ago I asked for information regarding bee nest blocks that work. Several folks responded with suggestions and links. (Thank you!) After a lot of reading I found what appears to be the best solution. I've written about it at my blog: http://www.beementor.com/the-best-bee-nest-block/
All of this is based on second-hand information still. Hopefully after this year I'll have some of my own observations to share about the effectiveness of this nest block.
If you'd like a list of the sites and documents that I visited and read, let me know and I'll compile them for you.