I've had a great time watching videos that explore the science and engineering behind the James Webb Space Telescope. It's an incredible feat of engineering that should lead to a good string of scientific discoveries... and also it's a vastly-over-budget project with way too many single points of failure and significant opportunity cost if you assume a fixed NASA budget.
I'll be very interested to see how much the budget of projects like the JWST could come down as we develop low cost ultra heavy launch capability and better space-based robotics that simplify servicing and upgrading.
Here's a great intro to the overall project and the science they hope to do: (via the always-compelling Scott Manley)
A more detailed intro, somewhat duplicative of the first one:
Here's a detailed video showing the long and complex deployment sequence that's fraught with challenges:
