The 2005 google algorithm update brought the efficiency of the search engine to the higher level. Spam via fake backlinks got a drastic reduction.

NoFollow attribute: [January 2005 google algorithm update ]

On January 2005, the search engines dominating the market such as Yahoo, Bing (MSN Network) and google joined hands together. They contributed towards the ‘NOFOLLOW attribute’. This new indexing web command ensured that no fake backlinks were created.
Earlier, spammers had the ability to comment their own website links onto the comment section of any high rated blog or forum or any website. This generated backlinks to the spammer’s webpage in turn enhancing the page rank. By this attribute, the links on the comment section carried a default NoFollow attribute. A NoFollow attribute nullifies the backlink detected by google and hence goes unaccounted!
This is not technically an update, but the introduction of NoFollow had a huge impact on link graph!

Allegra Update: [February 2005 google algorithm update ]

The Allegra update focused mainly on LSI [Latent Semantic Indexing] algorithms. Very few details were known about the tweaks in the indexing. Speculated rumors suggested that google started to penalize websites for fake backlinks from irrelevant websites.

Bourbon Update: [May 2005 google algorithm update ]

Bourbon update stretched over a week with several tweaks and enhancements. It was seen that 3.5 improvements in search results was mentioned during release. This meant that the improvements in search algorithm was not yet complete and it lasted for a week. It was speculated by the webmasters that google changed the way non-canonical URLs were treated.

Personalized search and XML site maps: [June 2005 google algorithm update ]

• Personalized search was the next attempt by the search engine giant. When a user gets logged in, the web history of the user profile is reviewed. Websites that were already visited by the user gets high priority in search results.
Note: This is for that particular google account only. Once logged out, the personalized search effect goes off.
• After introduction of XML site maps, google had a very clear knowledge of the navigation methodology of every website. Users have an option either to submit the XML sitemap of their webpage directly. If not, worry not! Google bots and crawlers are always on its way spanning the internet. Google bots create an xml sitemap and submits it to google. The site navigation was given high importance as better navigation makes the user more comfortable.

• The change and rate of change of XML sitemaps were used to predict the core activities of the website.

Gilligan update: [September 2005 google algorithm update ]

The Gilligan update was rather termed as “False Update”. Mattcutts, the then head of anti-spam team of google mentioned that search algorithms had no major update. The Gilligan update was all about the toolbar PR that gets updated once in 3 months or so.

Jagger Update: [October 2005 google algorithm update ]

The Jagger update made a clear impact on fake backlinks. Jagger targeted low quality links – namely link farms, reciprocal links and paid backlinks! Jagger rolled out for a period of 3 months. i.e., Jagger got updates over a period of time fine-tuning the update. Removing unused pages, making a 301 redirect from non-canonical URL to canonical form of url, along with several tweaks were advised.

Integration of Google local and maps: [October 2005 google algorithm update]

‘Google Local’ was successfully launched. became a one-stop solution for mapping and local search [Location based search for nearby stuff]. ‘Google Local’ and ‘Google maps’ got integrated.
know more on Integration of google local and maps!

Big Daddy Update: [December 2005 google algorithm update]

The Big Daddy update was termed the ‘Infrastructure update’. Big Daddy changed the way of URL canonicalization, 301/302 redirects.