Three big places:
1 - Claude Desktop (and some more niche AI chat apps) - you can use MCPs to extend these chat systems today. I use a small number daily.
2 - Code Automation tools - they pretty much all have added MCP. Cursor, Claude Code, Cline, VSCode GH Codepilot, etc ...
3 - Agent/LLM automation frameworks. There are a ton of tools to build agentic apps and many support using MCP to to integrate third party APIs with limited to no boilerplate. And if there are are large libraries of every third party system you can imagine (like npm - but for APIs) then these are going to get used.
Still early days - but tons of real use, at least by the early adopter crowd. It isn't just a spec sitting on a shelf for all the many faults.
These examples seem to be tools built on this tool - which is cool and all, but it's not the equivalent of "a mail order catalogue you can access through your computer", or "a replacement for a travel agent".
What are the applications at the level of Amazon.com, Expedia, or Hacker News?