Well it depends on you preferance really. There are quite a few plants that can achieve this:
eleocharis parvula
riccia fluitans
any type of moss(ie. Java moss, Xmas moss etc)
glossotigma elatinoides
echinodorus tenellus
marsilea quadrifolia
Pygmy chain swords
dwarf sagittaria
I think there are a few I missed but that'll give you some ideas.