Another commercial AAC implementation, QuickTime AAC is known to be one of the highest quality medium-bitrate CBR LC AAC encoders. Whilst the current version only supports 2 channel audio, it is hoped multi-channel support, along with VBR encoding will be added in a future version.
Although this is a comercial implementation and part of the QuickTime Pro package, the codec is available for free through the iTunes Digital Jukebox
Update: It is rumored that the next major version of Apple's codec (included in iTunes 5.0, as a part of iLife 5.0 in Mac OSX Tiger to be released early in 2005) will support AAC HE & VBR encoding.
More information can be found at the Apple Website