Another good free one, always kept up to date:
http://lamexp.sourceforge.net/
Top of line professional one, £35:
http://www.sonnoxplugins.com/pub/plugins/products/codec/codectoolbox.html 

Probably a bit off topic to the question of the OP, but ill post it anyway..

Ehh why do it difficult, Export from sonar as usual to wave

then use Mediahuman Audio Converter 

if you download from youtube as well, it will convert the video to audio

Many formats supported, from they'r site :

MediaHuman Audio Converter is a freeware application for Mac OS X and Windows. It can help you to convert your music absolutely free in WMA, MP3, AAC, WAV, FLAC, OGG, AIFF or Apple Lossless format.

• Simple and intuitive interface
• Converts between all key audio formats
• Splitting tracks by CUE sheet
• iTunes support
• Converts multiple files simultaneously
• Keeps folder structure
• Extracts audio track from video
• Absolutely FREE!
  
  I use it much when needed,  and it is quick
  
  Jan
  
  
  

I don't ever wish to export directly to MP3 from Sonar. I always want to test a mix on other systems before committing to it. If it's ok, then I'll convert to MP3. Omitting the testing stage is like going into print without proofreading.

Spot on Kev.
 
I always export to wav first and then IF MP3 conversion is necessary, it's done outside of Sonar.

Bristol_Jonesey
Spot on Kev.
 
I always export to wav first and then IF MP3 conversion is necessary, it's done outside of Sonar.

Exactly. Same here. There is an option to keep the wav from memory after mp3 conversion I think though from Sonar.

I have used Traders Little Helper for converting WAV to MP3 or FLAC for years.  Easy to use and contains everything needed within the program and it is free.
 
http://tlh.easytree.org/
 
 

I developed a habit of using Audacity or SoundForge.  I export as a WAV out of Sonar (or whatever), then drag/drop the WAV onto Audacity.  It's so quick and simple to trim, fade in/out and dump an MP3 from there it should be illegal.
 
Audacity might not be good enough for PRO use though

I only create the exported audio to MP3 files to shoot compressed quick and dirty 'in progress' versions to friends or other participants from a given project, as they are easier to send to an email address than uncompressed wave files.
 
Bob Bone
 

With sites like box and dropbox, the need to email audio feels is less and less.  With memory and disk storage getting dirt cheap and cheaper all the time, I don't see people tolerating destructive formats much longer anyway.
 
I just want my 16Meg audio player to hold more than 1 or 2 tunes.  I guess I need to replace that one day ...

Personally I hope MP3, 32 bit plugins and ASIO4ALL die a miserable painful death. There really is not much use for mp3 any more, even with cellphones. Wav is what you should master to, and FLAC is there for compression requirements.