one well known one that is called MB trading. I am unclear about whether this refers to stock trading or to forex markets or both. Can anyone help me out?

Answer
MT4 - metatrader allows you to write expert advisors - it's not really an api but more like a script...

Do you know an open source electronic trading platform similar to MetaTrader?

I'm looking for an open source alternative to MetaTrader (http://www.metaquotes.net/metatrader/) that can run on the Linux platform.

Answer
It appears that the API is open source.
http://www.metaquotes.net/metatraderapi/

I'm not familiar with the program and can't seem to find an alternative with just a quick search.

Have you tried running it in WINE?
http://www.forexhowto.com/blog/linux_charting_application/

You may also want to check this forum discussion:
http://www.forex-tsd.com/general-discussion/2354-how-linux.html?highlight=linux

One of the posts there mentions crossover, which is basically the commercial version of WINE that includes proprietary code. But it appears that you should be able to run this with just the open source WINE.