There are quite a few species;
Bacopa australis is very easy to keep, but plantlets need to be planted fairly close together as the plant does not spread as well as other harder-to-keep carpetting plants... info on link:
http://www.aquaessentials.co.uk/index.php?main_page=product_info&cPath=255_276_291&products_id=1216
Echinodorus Tennellus is always a good choice, and in my experience, it is fairly easy to keep... They do however, need high lighting conditions and CO2 to create a really compact lawn (but still a good foreground plant if planted fairly closely together...) You need at least 2WPg for this plant (and I would recommend nutrient-rich substrate...)
Eleocharis "parvula" is also a good foreground plant (hairgrass) and can be kept in 2WPG of light... If you plant it closely together then it forms a nice looking carpet...
I wouldn't suggest Glossostigma elatinoides or Hemianthus callitrichoides because they need fairly good conditions to grow properly (though a friend of mine grows HC very sucessfully in a setup of only 2WPG, fairly low CO2 level and nutrient-rich substrate... GE needs high light and CO2...
Pogostemon helferi is fairly easy to grow, but requires Co2 to look good...
And there is, ofcourse, Utricularia graminifolia (UG) which I have in my Nano! This plant takes quite along time to grow in to a carpet and, as I have found out, grows very vertically if not given enough light... Over 2WPG is probably better, but I keep mine in just under 2 and it seems to be growing well... I also add CO2 (Nutrafin fermenting kit and Seachem Flourish Excel...) Nutrient-rich substrate is pretty important as well... I have found that UG takes quite along time to addapt to new conditions and it takes even longer to properly root...
Thats just a small selection... There are plenty others (though they were the ones from the top of my head...) Visit the tropica website; they have lots of info on hundreds of plant species and pictures of every plant...
Cheers,
CJ...