My favorite is tree lounge. In my opinion it's the most comfortable and the safest on the market.
Only disadvantage is it's so comfortable you'll probably find it difficult to stay awake and it's a bit heavy compared to some I've tried. Check it out at
http://www.treelounge.com/
I believe Margaret talks on the forum about the only reported accidents involving tree lounge. Both were attributed to a bad supply of aluminum way back in the 80's or something. The aluminum supply company was found at fault in both instances.