The Nero AAC Encoder is a fully functional, high quality, commandline LC and HE AAC encoder and is free for non-commercial use. There are versions for both Windows and Linux (x86)
The most recent version is The same encoder is used in Nero 10 and Nero Move it.
The Nero AAC Encoder can be downloaded here. This package includes the encoder, a decoder and a MP4 tagging utility.
Usage information can be found in the readme.txt or by running each of the tools without specifying any options.

Results of AAC at 128kbps v2 public Listening Test
Results of 64kbps Public Listening Test
Results of the public multiformat listening test @ 64 kbps
48kbps AAC public test results
Results of the public multiformat listening test @ 48 kbps
Results of Dial-up bitrate public Listening Test

Questions and answers:
Q: What is the expected bitrate given a -q value?
A: The graph below gives an estimate for the bitrate that can be expected for every -q value. This graph is for stereo files only, files with different amounts of channels should scale linearly with the amount of channels.
Note that the table is only an estimate that can be expected on a large set of files. Single files may give very different results.

Q: What is the -2pass option good for?
A: Two pass encoding ensures that the output quality is constant (like VBR) while also ensuring that a specified output bitrate is achieved (like CBR).

Nero AAC usage information on the Hydrogenaudio wiki
Nero AAC recommended settings on Hydrogenaudio