What Chronometria said si correct. http://radegast.org/wp/
Is a chat only client for windows and linux. The developer just announced that they will no longer be supporting it but that it is open source. Word is it still works and it is on the approved SL client list.
Then you have the phone clients. Lumiya for Android. Pocket Metaverse for iPhone.
There is also a new software server client like SLGo was until it got bought by Sony. https://www.brightcanopy.com/
It is a web interface that you run the SL viewer or Firestorm viewer via a web page. The viewers are in their cloud. They got overwhelmed when it opened last week. It is USD$17 a month. You can get to the website and run on WinXP, Linux, old Mac, etc. Anything with a reasonably up to date web viewer.
I hope this helps,
Take care Ray.