I initially closed it under the assumption that you were looking for rankings of such sites, but really, at the question's core, you are asking for site recommendations. I adjusted the close reason accordingly.
Lists of anything, but recommendations in particular, go stale quickly. We've found this firsthand with many of the recommendation questions that we had on this site before the requirements for questions were changed. As such, I don't think there's any additional level of detail that can be added into the question to make it on-topic. Answering particular question about existing web applications is really our forte here.
In terms of migration, I don't think this is on-topic anywhere on Stack Exchange.