Does anyone know what this pretty little plant is?

Here’s a better picture of it, in the pot where it volunteered. Click on the image for a larger view.

A flurry of these things popped up in the El Niño rains this spring. I thought they were weeds and was about to extirpate them, but the seedlings are so pretty I relented and decided to see what they’d do. As fresh young babes, they have striking foliage: deep green trimmed in red. Here’s some that volunteered between the flags in the courtyard.

When they were younger, they had more red on them. It’s hard to see, in this picture, just how much red they have around the outside of the leaves and the stems.
Anybody know what it is?
