I’m a big fan of Fridays, not just because the weekend is close, but Friday is the day SparkFun releases new products. I have a special box near my home work bench with all the breakout boards I’ve purchased through the past few years. I have a break board for almost every purpose. Even the chinese are seeing value in producing breakout boards for smaller SMD components. You can find a number of interesting modular pieces on ebay through other Hong-King storefronts.
The state of industry (and market demand) is now advanced enough that kits are being produced using several different breakout boards. Take for example the FabFM kit. It uses a SI4703 breakout board. Anyone could modify such a kit with extra features, but it also looks aesthetically pleasing.