The first is Open Source Mathematics. This site is devoted to sharing Open Source mathematical software and developments. I would hope that this site develops as this is an area I would like to see increase greatly. It seems the site owner is definitely a proponent of Sage. That is a good segway to my next link.
The next link is Sage. Sage is an open source mathematics software system. Sage is very similar to Matlab, Mathmatica, or NumPy. From the Sage website...
Sage can be used to study general and advanced, pure and applied mathematics. This includes a huge range of mathematics, including algebra, calculus, elementary to very advanced number theory, cryptography, numerical computation, commutative algebra, group theory, combinatorics, graph theory, exact linear algebra and much more. It combines various software packages and seamlessly integrates their functionality into a common experience. It is well suited for education, studying and research.
Be sure to look at the Sage tour for more information and functionality.
Next link from DevelopSpace.net is Open Source Engineering Tools. As you can guess there is a variety of open source tools for the Engineering practioner. Unfortunately it is sorely lacking in mathematical and numerical applications. I might volunteer and help out where I can. I am sure they would want the extra assistance. As an aside here is a Linux.com article about DevelopSpace.net and their concept.
The last link is Opennovation.org. A self described open source engineering portal for news, links, resources of open source engineering software. The list is very complete for most types of engineering disciplines. Although I would like to see more Industrial Engineering at this site as well. Very good and looking forward to seeing more developments at Opennovation.org.