You don't say what kind of effect you want - formal or otherwise. Is the whole square edged with boxwood or just the "X" making 4 planting areas? Florals, herbs, vegetables in the spaces? If the boxwood is kept low to allow light, any plant suitable for your zone would work for you depending on the look you want.

Herbs have traditionally been the favored plants for knot gardens. I don't do herbs and don't know a lot about growing them in the garden, but there are endless entries with images on Google for knot gardens and knot garden plantings. But when choosing plants, consider the texture, size, and color of leaves and flowers as well as the plant's growth habit.

A few good choices might be - Hidcote or Munstead lavender, "Silver Mound" artemisia; Nepetas; some of the Veronicas; succulents like Hens & Chicks; sedums. A bit of bias as I tend to like the grey shades to compliment boxwood and be a little lean on the florals... unless doing a carpet bedding design - in which case I'd fill the spaces with annuals so color schemes could change from year to year as whims dictate.

The square is not edged with boxwoods, just the X, and there are 4 planting areas. The boxwoods have grown together to form a hedge at this point.

I was thinking a more formal design. I like the lavender idea. I already have a bed next to the knot garden devoted to herbs. Silver mound is a great idea too!

Hosta Lillies? ;o)

Here is a link that might be useful: Hosta Lillies