I thought I saw a thread on this but searching did not give me any hits. What alternatives are there to emulate/simulate/sample Mellotron voices? I know about Sampletron. What else is there? I apologize if this is in the wrong forum.

Here is a free sample one that as the descriptions says it is  purposely degraded.
 
http://www.tronsonic.com/products
 
Also there is UVI Mello which is offered at discount at times.
 
The Kontakt  library also has some samples in it if you own that. 

http://www.gforcesoftware.com/products/m-tron-pro

A few freebies are Tapeworm, Tapeotronic and Redtron.

Hollow Sun's Newtron Bomb is the main competition to GForce M-tron pro.

Don't forget the Kontakt factory library has a few.  There's also the eSoundz Infinite Kapsule which has a good variety of tron sounds as well.
 
If you want to check out some alternatives, I'd first suggest the Optomotron by Hollow Sun.  It's based on the Optigan, which is like a Mellotron except that instead of tape the sounds are stored on an optical disc (a sheet of plastic with some markings on it).  Also their MLM series have a few titles with some seriously crusty string, brass, and choir sounds.
 
One other thing to check out is the VP-330 (also Hollow Sun).  A lot of prog artists used to take that on the road as a replacement for the choir and string sounds because Mellotrons are not known for their roadworthiness.

Digital Sound Factory / Cakewalk do a Mellotron pack for Sound Center:
www.youtube.com/watch?v=PZb71XPJIJ0
 
Somewhat 'tweakable' by the looks to create your own variations.
 
Note: Sound Center is only 32bit, but seamlessly BitBridges to Sonar/64bit.

The DSF Mellotron pack was recently on sale for $10. Not bad at the price, especially after moving the sfz+samples into Rapture. Another free option http://forum.cakewalk.com/FindPost/1410821

scook
The DSF Mellotron pack was recent on sale for $10. Not bad at the price, especially after moving the sfz+samples into Rapture. Another free option http://forum.cakewalk.com/FindPost/1410821

Nice one @scook ... I've not actually thought about migration to Rapture before ... massively 'tweakable'.

I've got UVI's Mello that I use with MOTU's Mach 5. I know there were some issues folks here experienced with it and it requires an iLok (all of MOTU's and UVI's stuff does, I think). I actually really like it. I got it for free on a promo a few months ago. It's $99 at the UVI site and works with the free UVI player. I probably wouldn't pay full price but it's a good Mellotron.
http://www.uvi.net/en/vintage-corner/mello.html