IE9's release candidate, or RC, features faster performance, a small memory footprint, new support for additional web standards and the opt-in Tracking Protection that Microsoft has touted as its answer to calls for a Do Not Track feature in all browsers.
Microsoft debuted IE9 last March with the first of seven bare-bones builds dubbed 'Platform Previews', then followed those with a public beta in September 2010. More than 25 million copies of the beta have been downloaded in the five months since then, Microsoft has claimed.
"With the Release Candidate, we've taken to heart over 17,000 pieces of feedback about IE9," said Dean Hachamovitch, the executive who heads the IE team, in a blog. "You will find the product has made progress on all fronts - performance and standards, user experience, and safety and privacy."
The enhancements and additions to IE9 RC include the roll-out of Tracking Protection, an opt-in tool disabled by default that relies on published lists to selectively block third-party sites and content embedded in Web sites.
Tracking Protection is Microsoft's response to growing concern on the part of consumers, privacy advocates and government regulators about online privacy, particularly how advertisers track users' movements and their purchasing habits.
The US Federal Trade Commission has said users should be able to block sites and advertising networks from following their movements online.
IE9's tool relies on lists created by others to allow or block a site or ad network from tracking users. (Microsoft has said it will not generate such lists.) Groups offering Tracking Protection lists today include Abine, EasyList, Privacy Choice and TRUSTe. A page on the IE9 site provides links to the lists that are available for download.
Although Microsoft has promised to complete IE9 this quarter, it has not set a launch date for the final version. Hachamovitch said only that the polished edition would "come shortly".
According to web analytics company Net Applications, IE lost more than six percentage points of user share in the past 12 months, and at the end of January accounted for 56 percent of all browsers, an historic low.
While IE8 now holds more than 34 percent of the market, the 2009 browser has been unable to stem the share bleeding. Google's Chrome, for example, accounts for nearly 11 percent of all browsers in use, an increase of 5.5 points in the last year.
IE9 RC can be installed on Windows Vista and Windows 7, but will not work on Windows XP , the operating system that continues to dominate the market.
The release candidate can be downloaded from Microsoft's website, or from the company's download center. The 32-bit version of IE9 RC weighs in at just over 19MB, while the 64-bit version is a 37MB download.
Microsoft also published a blocking toolkit for IT administrators who want to keep IE9 off company machines when Microsoft begins serving up the browser via Windows Update.
Microsoft rolls out such blocking kits - which include both executable scripts and Group Policy templates - prior to delivering major upgrades to its operating systems and browser through Windows Update.
Last November, for instance, Microsoft published a blocking kit for Windows 7 Service Pack 1 (SP1).