Astilbe, Marsh Marigold, Flag iris, Ligularia ...........

Scroll down to the bottom of the page and click on 'B' then on 'bog gardens' you will get lots of ideas... Some you could try are: Lysichiton americanus, Gunnera manicata, Primula pulverulenta and other candelabra primulas, Caltha Palustris, Rodgersia, Yellow Flag Iris, Osmunda regalis, Rheum palmatum and any of the Astilbes.

Iris Siberica grows well in the damp, it's one of my favourite plants, comes in a range of colours and is able to cope with boggy and wet conditions, grows to about 2 foot, flowers last a while as well, the leaves look good in contrast to other plants when it's not in flower.
It's quite cheap if you buy the bulbs from Aldi, Lidl or Wilkinsons, but is easily available in garden centres too.

If it is an area which you just want to fill and forget about, then why not plant the coloured-stemmed Cornus? Just buy a few different varieties and take cuttings, they root easily and then you will have an interesting area come winter time.