Some people have said cilantro, but I would say it has a much stronger, sharper flavor. Celery leaves are delicious and might be worth a try. I just started using a lot of coriander myself, and I think it would depend on the recipe. Another vaguely similar spice could be cumin, but I would use a smaller quantity since it tends to have a stronger flavor.
And speaking of the mild flavor of coriander… another option would be to leave it out completely. If it's a complex dish, you probably won't even notice it's gone. If it's truly important… go get some more. Can be obtained cheaply as “molido de cilantro” in mexican import stores, in small packages for around a dollar all over the US.