I've always used the Cakewalk MP3 converter.
Bought it years ago from CW.
Still works fine whenever I want an MP3 version of my mix.
 

I-Tunes has a great MP3 encoder that works great, sounds great, and best of all its free. Ive been using it for over a decade converting wave to MP3.

BooRazz
I-Tunes has a great MP3 encoder that works great, sounds great, and best of all its free. Ive been using it for over a decade converting wave to MP3.

the only problem is you have to install that bloat-ware bonjour.
I tunes is great for iTunes, and you are correct you can use its mp3 converter but in my opinion, installing that app isn't what you want to do on your digital work station.

Ok Charlie, Ill bite. What would be your best suggestion for an MP3 encoder considering I know nothing about Sonars MP3 converter?

The easiest way to export MP3 in SONAR is to buy the Cakewalk MP3 Activation which costs $9.99.
Once you buy this activation it can be used for all future SONAR versions.
The reason its not free is because MP3 encoding is still a patent owned by Fraunhofer. 
 
Edit: corrected link to newer sku for X series and beyond.

Noel Borthwick [Cakewalk]
The easiest way to export MP3 in SONAR is to buy the Cakewalk integrated MP3 encoder It costs $10.
The reason its not free is because MP3 encoding is still a patent owned by Fraunhofer. 

When I followed your link and added the MP3 encoder to my cart, It said it was for old Cakewalk products?

"Cakewalk MP3 Encoder Download. $9.99
MP3 Encoding for Old Cakewalk Products"

When I clicked on 'More Info' I got the information below...

"Additional Information
Activate the MP3 encoder that comes with Cakewalk software: use with SONAR 1-3, Pro Audio 9, Home Studio 9 & 2002, Guitar Tracks 2 & Pro, Pro Tracks, Club Tracks, Plasma, and Music Creator (except version 3-7)."

Is this MP3 Encoder compatible with the latest Sonar? Does it need an update? Or, at least an update to the documentation...

I don't need an MP3 Encoder but it would be convenient to have it available inside Sonar...

The link for SONAR 4 through SONAR Artist, Professional and Platinum is Cakewalk MP3 Activation

Thanks! B)

$10 isn't bad for convenience sake, if it works well and I'll be using it for years...

BooRazz
Ok Charlie, Ill bite. What would be your best suggestion for an MP3 encoder considering I know nothing about Sonars MP3 converter?

LAME http://forum.cakewalk.com/TUTORIAL-Getting-LAME-MP3-Encoder-to-Work-With-SONAR-m1255631.aspx

MPH
When I followed your link and added the MP3 encoder to my cart, It said it was for old Cakewalk products?

I'm sorry about that. I posted a link to the older encoder. I corrected my post with the right link that scook posted.