Compaact! is one of the newest AAC encoders. Like Nero AAC, compaact! is not free, however it does offer an impressive feature set. Roberto Amorim's last AAC test showed that at 128kbps, Compaact! is tied with both the FAAC and Coding Technologies (Real) encoders. Compaact! features both the LC and Main Object Types, CBR, VBR, Multichannel, high resolution (24bit/96kHz) encoding, and command line support.
For portable encoding, try -q5 to -q6. For music archive purposes, try -q7 to -q8.
Compaact! development has been recently discontinued, therefore, we have probably seen the last of it.