LAME MP3 Encoder is the best mp3 encoder and is available for free! LAME originally stood for L AME A in't an M p3 E ncoder . LAME is an educational tool to be used for learning about MP3 encoding. Features:

Many improvements in quality in speed over ISO reference software.
MPEG1,2 and 2.5 layer III encoding.
CBR (constant bitrate) and two types of variable bitrate, VBR and ABR.
Encoding engine can be compiled as a shared library (Linux/UNIX), DLL or ACM codec (Windows).
Free format encoding and decoding
GPSYCHO: a GPL'd psycho acoustic and noise shaping model.
Powerfull and easy to use presets.
Quality is comparable to FhG encoding engines and substantially better than most other encoders.
Fast! Encodes faster than real time on a PII 266 at highest quality mode.
MP3x: a GTK/X-Window MP3 frame analyzer for both .mp3 and unencoded audio files.

Changes from LAME MP3 Encoder 3.96 b1 to LAME MP3 Encoder 3.96 b2:

removed unnecessary integer convertion in resampling
reworked scalefactor allocation in vbr-new
fixed a freeformat decoding problem
updated minimal bitrate for V1 and V2
added ability to disable ReplayGain analysis

Download: LAME MP3 Encoder 3.96 b2