It looks like lettuce. Any chance you let things go towards the end of the season? Lettuce goes to seed as the weather warms up and these seeds likely are producing the plants you see now.

Oh and it's an annual, these are new seedlings you're seeing.

Err, if you don't know that this is lettuce, how are you competent to grow and harvest a garden. Perhaps you're eating weeds ? :) (no..seriously, this is scary, there are lots of potentially poisonous plants out there) I will however give you props to asking before eating it.
Or maybe I took this entirely the wrong way, and you're just astounded that it reseeded? If so, I'm sorry I jumped on a wrong assumption.

More to the point, if last year you didn't let your lettuce bolt, flower, and go to seed, it probably isn't lettuce, at least not the lettuce you planted. If you did, lettuce produces seeds efficiently, and it likely is. Not sure where you are, but I suppose if you have very mild winters, you might get some regrowth from existing roots.

Thank you for the helpful comments. I did let it go to seed at the end of the year. I didn't know much about growing veggies and didn't realize that you need to keep it all cut back to keep that from happening. It seems though that it may be worth letting a few of them go to seed in order to seed for the next year?

I agree, letting it go to seed, usually is just a cost-saving measure and makes perfect sense to me, though collecting and sowing usually works better than just letting them self-sow (unless you're very lucky).