FAAD2 2.0 is now available as Release Candidate 1. The most important new feature is the support for the HE AAC profile (HE=high efficiency).
HE AAC is the joining of MPEG4 AAC with SBR - the well know technology used in MP3pro - to create the perceptually best audio encoder for low bitrates (less than 80kbps)
Formal listening tests conduced by the MPEG demonstrate that HE AAC offers excellent quality for non-critical listening on bitrates around 48~64kbps.
The sources are licensed under the GPL and are available as a convenient library. Also included are sources for a command line decoder that can decode both AAC inside the MP4 container and unwrapped AAC; plugins for several media players (including Winamp and XMMS), a DirectShow filter and several other tools.