One of the biggest is a drive-wiping tool that can wipe all the data from your hard drive, but can also scrub only the available free space. As with many of the tools in CCleaner, it's fairly nuanced and allows for a simple one-pass overwrite and three levels of secure deletion. These include a Department of Defense-level three-pass option, a National Security Administration-level seven-pass cleaning, and a 35-pass Gutmann-level deep scrub. Note that the more passes you select, the slower the deletion process.
Another key improvement to CCleaner 3 has been adding more options to pre-existing features. You can now select specific Internet cookies to keep, across all your browsers, while CCleaner deletes the rest in Internet Explorer, Firefox, Chrome, Opera, and Safari. The internal scanning tech that powers CCleaner has been improved, according to Piriform, and the interface received some minor tweaks. There's also a new native installer for x64, and environment variables have been added for %SystemDirectory%, %SystemDirectory32%, and %SystemDirectory64%.
Version 3 supports more Windows programs than before, including added support for Microsoft Silverlight Isolated Storage, AVG 2011, Audacity, LogMeIn Hamachi, BitTorrent, and Windows Game Explorer. Pre-existing support has been improved for Google Chrome, Internet Explorer 9 beta, and the torrent managing client Vuze.
In empirical testing, CCleaner 3 appeared to be marginally faster than previous versions. This is probably system-dependent, so users with older computers could likely see significantly faster scanning and cleaning times than in previous versions.