/r/buildapc and /r/buildapcforme. Has been helpful to get a general idea of what parts to get and feedback on if the build is balanced for what the user intends.
/r/buildapcsales, /r/gamedeals, /r/consoledeals are other ones. There's sites like isthereanydeals and pcpartpicker, but those subs have come in helpful for catching deals I would have missed.