Opencode

Of the ones I've personally used, Cline and RooCode are open source:

https://cline.bot/ https://roocode.com/

(although they offer many more configuration options than Copilot, I still find Copilot more convenient to use, but it's my personal opinion)

Also, I recently shared a link to coding agent rankings here:

https://lemmy.ml/post/45706361?scrollToComments=true

so you might consider trying out those popular ones.