Yasu Pallas,

The picture is not distinct enough for me to identify for sure, but it almost looks like a piece of a succulent milkweed dropped off of a mother plant and fell there - they do that all of the time and root easily.

You'll want to soon separate those plants, as they, from what I can see (a Euphorbia, an Opuntia and a Haworthia), have different cultural requirements. A few pictures which show your whole arrangement (which looks quite healthy) would help us ID them.

Opuntia - cactus
Haworthia - succulent, not a cactus
Euphorbia - succulent, not a cactus, but many look like cacti

This post was edited by cactusmcharris on Mon, Nov 4, 13 at 8:44

The little tike looks like possibly a Huernia, which may have come from seed (not from any plant pictured). Otherwise, ditto everything Jeff has said above. =)

Pallas,

What Rosemarie says is also a certain reality - Huernias and others like it can release thousands of seeds-with-wings that helicopter through the atmosphere, only to land and grow almost anywhere.