Big Daddy - December 2005
Technically, Big Daddy was an infrastructure update (like the more recent "Caffeine"), and it rolled out over a few months, wrapping up in March of 2006. Big Daddy changed the way Google handled URL canonicalization, redirects (301/302) and other technical issues.
Indexing timeline (MattCutts.com)
Jagger - October 2005
Google released a series of updates, mostly targeted at low-quality links, including reciprocal links, link farms, and paid links. Jagger rolled out in at least 3 stages, from roughly September to November of 2005, with the greatest impact occurring in October.
A Review Of The Jagger 2 Update (SERoundtable)
Google Local/Maps - October 2005
After launching the Local Business Center in March 2005 and encouraging businesses to update their information, Google merged its Maps data into the LBC, in a move that would eventually drive a number of changes in local SEO.
Gilligan - September 2005
Also called the "False" update ? webmasters saw changes (probably ongoing), but Google claimed no major algorithm update occurred. Matt Cutts wrote a blog post explaining that Google updated (at the time) index data daily but Toolbar PR and some other metrics only once every 3 months.
What?s an update? (MattCutts.com)
Personalized Search - June 2005
Unlike previous attempts at personalization, which required custom settings and profiles, the 2005 roll-out of personalized search tapped directly into users? search histories to automatically adjust results. Although the impact was small at first, Google would go on to use search history for many applications.
Search gets personal (Google)
XML Sitemaps - June 2005
Google allowed webmasters to submit XML sitemaps via Webmaster Tools, bypassing traditional HTML sitemaps, and giving SEOs direct (albeit minor) influence over crawling and indexation.
Bourbon - May 2005
"GoogleGuy" (likely Matt Cutts) announced that Google was rolling out "something like 3.5 changes in search quality." No one was sure what 0.5 of a change was, but Webmaster World members speculated that Bourbon changed how duplicate content and non-canonical (www vs. non-www) URLs were treated.
Google Update "Bourbon" (Batelle Media)
Bourbon Update Survival Kit (SERoundtable)
Allegra - February 2005
Webmasters witnessed ranking changes, but the specifics of the update were unclear. Some thought Allegra affected the "sandbox" while others believed that LSI had been tweaked. Additionally, some speculated that Google was beginning to penalize suspicious links.
Nofollow - January 2005
To combat spam and control outbound link quality, Google, Yahoo, and Microsoft collectively introduce the "nofollow" attribute. Nofollow helps clean up unvouched for links, including spammy blog comments. While not a traditional algorithm update, this change gradually has a significant impact on the link graph.
