When Software is free and its source code is open, developers join existing projects and collaborate. "Re-inventing the Wheel" becomes a rarity and existing projects grow and mature rapidly into a well polished product available-to-anyone. Web Projects can gain a Head Start by building off of Free and Open Source Software (FOSS).
As FOSS projects mature, they quickly gain a whole community of contributing developers watching and testing the code for bugs and security holes. Since the code is open, there are no secrets and anyone can keep an eye on it, anyone can submit patches and fix bugs. Its transparency is its security.
It also makes a Web Project more reliable. The software code is not at the mercy of one company. If whoever is working the project becomes unavailable, one has the entire FOSS community to turn to for free and paid support. Again, the code is open and there are no secrets. People can study, test, and use it freely. Out of this FOSS support companies flourish.
The software is FREE. That's almost enough right there. A Web Project can lower cost substantially by cutting out one of the biggest expenses: Development of Software or Licensing existing Solutions. FOSS is already available and free to use and modify as you see fit.
For Better Cheaper Web Developement, be sure to evalute FOSS for your next project.