This has appeared in our garden.
After some Googling, I figured out that it's Portulacaria Afra, also called Elephant's Food, also (aptly) called Red Stem. I was glad to find that it's not a weed - I think it's pretty. It does apparently get quite large - though it's also grown as bonsai, so it could be trained.
Our front garden has no shade at all, so it needs plants that can hold up to the beating sun and the heat and humidity of the summers here. I think I'll keep this guy around.
Since we just moved into our new house in mid-June, I'm only focusing on the yard basics this summer. Next year I'll have some grander plans for the gardens, etc.