The best replacement for mef is unity.

The best alternatives to mef are rated as following:
★★★★★  unity
★★★★  prism
★★★  ioc
★★  maf
★★  ninject
★  castle windsor
★  dependency injection
★  structuremap
  addin
  autofac

Your alternative for mef is not listed? Suggest!
Speaking about mef, one probably shouldn't forget about mvvm.

Other search terms related to mef and their ratings are:
★★★★★ mvvm
★★★ mvvm light
★★ autofac
You have in mind a companion for mef and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of mef in all years. Click on graph for more.
mef
unity
prism