Opium poppies are prevalent here, perhaps the most common reseeding annual. Sweet alyssum also does well.

Cleome, larkspur, annual bachelor's buttons, sweet peas and all annual poppies (includes Shirley, corn and California as well as the opium or breadseed poppies) are all very easy. Marigolds, cerinthe and nicotiana are also pretty easy. With the exception of the sweet peas, most will continue to reseed freely, too.

I don't grow many annuals, but nigella Miss Jekyll and papaver somniferum are always welcome. They both self sow regularly, sometimes I'll gather the seeds and toss them into a different area than they've chosen to sow themselves the previous year.

Occasionally a few sunflowers, but I really don't have the space - which reminds me, I should pick up a pack seeds of the giant types for a neighbor who does have the space and I can enjoy those from my driveway :)