1. Inside Search – Official Google Search Blog
The Inside Search blog is undoubtedly your most important source to monitor for news on the most recent algorithm updates and just what Google is attempting to target as such.
It is important to keep in mind that what Google attempts to do in the search engine results and what their algorithm update actually does are often two very different things. But more so than this, looking at what Google is targeting is great insight into what they deem as most important and/or major issues on the web currently.
2. MozCast – Google Algorithm Weather Forecast
When you are looking for the daily impact algorithms have on the web in general, MozCast is a great source. Every 24 hours the website will audit 1000 keywords to determine which top 10 results are showing that day for the query. They will then take those top 10 current results and compare them to the day prior to see just how much shake up has occurred. The bigger the change in results the higher the temperature.
This is a great way to collect a fairly small sample of search engine results and identify, in general, what is happening in Google currently due to algorithm changes.
3. Matt Cutts Google+
Matt Cutts is head of the webspam team at Google. He has become a type of spokesperson for the organic side of Google search results. He will often post each time a major roll out occurs in the algorithm and ask businesses to provide feedback on how or if they are impacted by the update. He will also typically announce how much of the web they estimate will be affected.
The unique element you will get when following Matt Cutts is that, at times, he will announce upcoming algorithm updates and elements they will be targeting in advance. This can be a great help in determining what your online strategy should be as you look into future months.
4. SEOmoz Google Algorithm Change History
For a comprehensive perspective on all public and documented algorithm updates there is no better source I have found than the SEOmoz Algorithm Change History.
Truly a beautiful peace of link bait online. But more than just link bait, you will find here the date, name and details of each algorithm update that has occurred dating back to 2000. You will also find additional sources for each update to help with digging into the details.