It was .ly, then it became .co, and now it's .io.

Note, though, that the implicit "I/O" reference in .io means that it's more often used for products aimed at a technical audience. So as a replacement for .com, I'm not really sure what the frontrunner is now.

A few other trends that are similarly related to the .com shortage: adding "get" to the beginning of the domain or "app"/"hq" to the end.

We're probably at the end of the "everyone wants a .com/.co/.io" era though. People will start to use TLDs that have semantic value ("beer.club" instead of "beerclubapp.com" for example)