From Hydrogenaudio Knowledgebase
aacplusenc is a multiplatform HE-AAC+ encoder.
It was written by Matteo Croce that tooks the 3gpp reference code and wrote his own routine to read wav files,
write RAW ADTS output as well as faster buffered I/O and custom FFT routines.
It hasn't fetures found in other encoders like VBR or 2 pass encoding, but it is fast and quality is quite good.
Also, it's the only HE-AAC+ encoder available on certain combination of Operating System/architecture like MacOSX, Linux/PPC etc.
Full source code is available for download on the official site.