CrossTalk is a chat server that aims to implement as many messaging platforms as possible, and allow them to (mostly) seamlessly federate with one another. It's currently in semi-open alpha. If you want access, join our Discord server linked above, then DM 623_chimaera
on Discord for instructions.
NOTE: This service and website are alpha-quality software. There are going to be issues. If you found any and want to report them (or just generally want to suggest a change/new feature), you can do so in our Discord server.