I'll start this off with one I "created" today. I have been having trouble finding my aunt and uncle on the 1930 census for St. Louis (and I "KNEW" that they were living there then!) — I was only 2-1/2 so the reconstructed memories are iffy. No search engine found them.
Today, I used the census district descriptions I found on Ancestry.com to narrow down the area to be searched. This list gives the boundaries of each enumeration district. I used boundary streets to find an appropriate district, and then searched the district to find the probable block of the probable street. Found them on the first try!
Obviously this only works if you have an idea of the appropriate area; but many of us do. My family was on Ancestry's page "22 of 78" for District 26 of 385 districts. I needed to look at address numbers and boundary streets for 21 pages before I found the useful block, but I only needed to search the actual street addresses on the one page.
Even if I hadn't been quite that lucky, I would still have been able to limit my search to the other side of that street and the remaining blocks of that street before I either found them or proved that my memory was faulty. I think this is a real time-saver.
Sue