Not one I know except from pics, but compare it to coffee, Coffea botanically. Coffea arabica is the usual species sold as house plant, but ya never know when someone will start selling something they didn't before.

Thank you all for the help identifying this. I'm thinking I'm going to try to get rid of it - would garden experts agree that this is one I shouldn't feel bad about getting rid of? FataMorgana- when you said the early leaves don't always look like mature leaves.... Any chance that my giant collection of "little spreaders" that is popping up with on pairs of opposed leaves is just a pre-woody-vine version of this? Thank you all again for the help- one down five hundred to go! :-)

The hitch-hiker looks like something from Crassulaceae family. If it's making pairs of opposite leaves, Crassula genus is likely. Once it gets some age in better light, it should become recognizable. Cute!

Very pretty! Maybe Belladonna? I haven't grown it, but someone generously passed along some bulbs when sending some cuttings so I'm looking forward to having it grow and bloom in my yard. This is what's fun about plant swaps. Something comes up and you think "Where in the world did that come from?" That's happened to me many times.