Some men are sponge-worthy, and some men are not sponge-worthy. Sponge-worthy is a phrase used by Elaine about a man who is worthy of having sex with and thus "using up" one of her contraceptives. The contraceptive sponges Elaine uses has gone out of production.
This video is showing some very sponge-worthy scenes. George wants one because he is on the verge of makeup sex with Susan (and Susan loves the sponge), but Elaine has not a single one to spare, even if she has "cleaned out the whole west side".
Read more about contraceptive sponges on Wikipedia