I'm trying to find Greenland road or main trail maps. Also, if there are ferry lines that join Greenland cities it will be very useful. Any link?
For town maps, Greenland.com has interactive online maps. Most towns will also have a free printed tourist map.
Arctic Umiaq Line provides in-season ferry service on the west coast.
And don't be too down on OpenStreetMaps. The reality in places as remote as most of Greenland is that the printed maps have the same problem as OSM: there may be roads that aren't on the map, and things on the map that have been under the permafrost for 20 years. These things happen when you're a "road less traveled" kind of traveler.
I'd definitely be downloading the OSM maps of Greenland to my iPhone as a just-in-case backup. Check out the app Gaia GPS, which will enable you to download maps for offline use from both OpenStreetMaps and Cloudmade, giving you further comfort since you have two different maps to compare.
Wear warm underwear, and have fun!