It does not look like a sunflower to me. It could be a tomato. Those first leaves sometimes look like that. I'd let it grow a bit longer and see what happens.

IT's not a grass or any other monocot--those come up with one leaf--this is a dicot. I don't think that is a sunflower--the base of the leaf close to the stem isn't broad enough. Sunflower seedling leaves are shaped very much like a spear point.

If you used bagged soil for starting the tomato seeds these weed seeds may have been in that soil. It happens every now and then, I think most bags of soil have a weed seed or two or three dozen, lol. If you used locally produced soil then it's likely to contain a local weed seed. I'll paste in a link.

Here is a link that might be useful: eWeeds from Valencia College

I'm always searching for good weed and wildflower sites ... but I'm not too sure about some of those weed identifications in the first half of that page.

Their photo of "Spurge" is definitely Purslane. The two don't look at all alike to me, but apparently many people have devoted blog posts to distinguishing the fleshy, edible Purslane from the non-fleshy, milky-sapped, toxic Spurge (this one looks like Spotted Spurge to me; when I was a teen and the commonest form of child labor was Weeding the Lawn, we used to call it Spotted Scourge). [Purslane is shown twice in the second half of the page: I'm not so sure about the first of those two -- perhaps a variety I'm not familiar with -- but the second is definitely Purslane.]

The photo of Broad-leaved Plantain in the first half isn't; I assume it's a second Hawksbeard photo. Both Broad-leaved and Buckhorn Plantains appear in the second half of the page.

The photo labelled Kyllinga could be anything. Looks like plain old grass to me -- but they are sometimes difficult to tell apart. Unfortunately, I'm all too well acquainted with Green Kyllinga, one of three Nutsedges I've identified on this property.

missing, you need to contact that site administrator and let them know there is a problem with some of their IDs. If you have links to other confirming pages then include them in your email, I think they would be appreciative.

T--happy that is not my site.

Here is a larger plant of the same group. Now I am really curious. I think it is the result of bird crap.

I'll try, Trudi. But it's usually an exercise in futility. I studied English and Library Science, not botany or entomology -- though most likely I'll just be ignored because the site admin's too busy, the site's no longer active, or the contact info has expired.

garf, have you tried posting the photos on the Name That Plant! forum? Lots of incredibly knowledgeable people there.
http://forums2.gardenweb.com/forums/namegal/

Ok, definitely not a tomato. Not sure what it is other than a "weed".

missing, see if they have an agricultural cybrarian.

T