Innovator's Patent Agreement
= any patents obtained by employees at Twitter (past or present) grant lifetime control to the actual inventors, to prevent the patents from being used offensively against others. 
From the Twitter agreement:
"One of the great things about Twitter is working with so many talented folks who dream up and build incredible products day in and day out. Like many companies, we apply for patents on a bunch of these inventions. However, we also think a lot about how those patents may be used in the future; we sometimes worry that they may be used to impede the innovation of others. For that reason, we are publishing a draft of the Innovator’s Patent Agreement, which we informally call the “IPA”.
The IPA is a new way to do patent assignment that keeps control in the hands of engineers and designers. It is a commitment from Twitter to our employees that patents can only be used for defensive purposes. We will not use the patents from employees’ inventions in offensive litigation without their permission. What’s more, this control flows with the patents, so if we sold them to others, they could only use them as the inventor intended." (http://www.techdirt.com/articles/20120417/10324218529/twitters-revolutionary-agreement-lets-original-inventors-stop-patent-trolls.shtml)
"As Twitter notes, this is "a significant departure" from how just about every other company handles patent assignments. Along those lines, it's planning to evangelize this idea to other tech firms as well -- and I wouldn't be surprised to see a bunch of others jump on board. The basic idea makes a lot of sense. Twitter has also posted the full agreement to Github and put it under a Creative Commons license.
The method by which this works is pretty creative. Basically, if the actual patent holder tries to use the patent offensively without first obtaining the permission of the inventor, the agreement allows the inventor to issue a license to the entity being sued." (http://www.techdirt.com/articles/20120417/10324218529/twitters-revolutionary-agreement-lets-original-inventors-stop-patent-trolls.shtml)