Modular furniture is a well-known concept; like LEGO, you can move pieces around to fit your living room needs du jour. Modular gadgets are less familiar and not as common, but they're slowly making waves in the technology industry.
The idea is just as viable for phones and computers as it is for sofas and tables -- the ability to move parts to accommodate changing needs reduces e-waste (only produce what you need), makes updating devices easier (only replace or upgrade one broken component), and saves energy (only power what's being used).
Modular technology is a young field. Most of the current technology applies to basic robotics and computing, not consumer products. Here's a tour of what those look like, what's on the market today, and, most fun of all, what designers and scientists have dreamed up for the future.