My favorite (out of two
) plain t-shirt is from Japanese weekend. I got it on clearance and it was really about evenly priced with a normal Old Navy maternity t-shirt.
That said, I also really like the plain ol' black maternity t-shirt I have from Old Navy. In my opinion, their sizes run large-- but I've heard other people say that they think they run small. So if you CAN get into an Old Navy that actually has maternity clothes in it, it might be worth going just to try stuff on.
My favorite "nice" shirt was also on clearance at Japanese Weekend. It's one of their combo maternity/breastfeeding tops, though, and I am pretty sure I'm not going to be able to wear it all the way through pregnancy... it just doesn't stretch that much. It is awfully nice looking, though, and better than other crossover-y BFing tops I've tried on other places. (Really didn't like the ones at Motherhood.)
Motherhood's overpriced for what it is* (IMHO) and their return policy sucks, BUT if you have an actual store near you, they often have good stuff on clearance and their clearance prices are fine.
*I should note that I think more or less ALL maternity clothing is overpriced for what it is.