I dont have any window boxes but when i visited my local garden centre yesterday the bedding plants that caught my eye where pansies and also these little daisey type flowers think they might of been Bellis, there were more but not sure what they were. I'm sure these would be fine, also try ivy, Skimmia,heathers and cyclamen. Hope this helps.

It also partly depends on the size of the window box. Little 15 cm wide and deep ones will only work with the very smallest annuals and hardy succulents. 30 cm wide and deep ones, though hard to hang securely, will grow everything up to small shrubs.

I have seen very effective semi-permanent tub and window-box plantings using miniature conifers and evergreens such as eleagnus and choysia , with winter pansies/violas added for colour, and trailing ivies for shape, all underplanted with bulbs such as muscari and miniature daffodils and tulips and small white cyclamen (these last won't survive once the weather turns really cold). It depends on how big your container is, but this combination can look good right through until late spring planting is appropriate, or even right through the year if there is room to add some summer colour later on.

Tumbling rosemary and thymes will work for a while and can be used. A little alpine window-box could be nice.

Try a mix of foliage and flower. If you can get them still, i'd use varigated ivy trailing over the front, a nice lime green goldcrest conifer in the middle as a focal point, some cineraria silver dust to set off the colours and then interplanted with pansies. It is also possible to add a few small pots of bulbs such as crocus and narcissi. Nothing wrong with adding some smaller herbs as mentioned above too such as thyme or golden margoram.