When you enter your family tree at Ancestry.com (with a setting either as a Public or Private Family Tree) Ancestry will provide what they call 'hints' when they see a record anywhere in their databases they think matches one of the entries in your tree.
When I first entered my family tree I wasn't really impressed with this, because I'm pretty good at searching the databases, so it wasn't turning up things I hadn't already discovered. However, there are two factors I didn't consider.
1) Most obviously, Ancestry is constantly adding databases, and *should* add the hints pretty quickly after new databases are added. So paying attention to any new hints should reveal sources in new databases.
2) Ancestry's algorithm for hints catches more than my normal searches would.
Not that I can improve my searches much, either, because I am a human, and not a computer, and I don't have the time to go digging through random databases.
For example, naturally, I have entered first and middle names of individuals in my family tree where I know them. However, if in the database they are listed by their initials and surname only, I'm not likely to catch them in my Ancestry-wide searches. If I am searching a particular database I know they have to be in, I'll get more creative. I'll even search by surname alone, but while I don't have 'Smith' as a family surname, many of them are common enough to make that method impractical across the entire site.
But browsing for hints, I saw the little Ancestry hint leaf flashing for my mother, and I found that odd, because few databases exist where her records are available yet. She'll be in the 1940 census, but we have to wait until 2012 for that.
However, states sometimes have censuses in off years, and I discovered that there is one state that has already released their 1935 and 1945 census - Florida.
I didn't find it with my normal searches on my grandparents because everyone on their page was recorded by their surname and initials. Ancestry's Family Tree software found it though. (Other pages of the census have given names recorded, it appears a particular census taker got lazy.)
I knew my grandfather had been stationed in Florida at the end of the war, and his wife and children had come out to join him. It was for a brief period of time before they returned to St. Louis, but it happened to be the right time for the 1945 Florida census.
There was one interesting historical note about the 1945 Florida Census
In the upper right hand corner of the page was "Race" for the entire page (covering 49 people). Which meant Florida not only knew there wasn't a single neighborhood in the entire state that would be mixed, they decided to take advantage of that to make the census form have one less column. (The 1935 census indicated race individually.)