^theres not really any "islands" in the game. the edges of the main map are as far as you can go in the game technically. you can access "point lookout" my going to the riverboat landing and taking the boat there once you get the DLC. same with The Pitt, etc, once the DLC is loaded you will be able to access these parts of the game from different locations that appear on the world map.
i would rank the DLC's as follows, however each DLC has new weopons, armor, etc that you can get
1. Point Lookout - great open world area, good story, kill hillbillies, discover wild punga fruit and refined punga fruit which is great for health,
2. The Pitt (has what I think is the hardest achievement in the game, getting 100 of the steel ingots. maybe not the hardest but its time consuming the find them all unless you go watch the youtube video on it and get them in one swoop)
3. Mothership Zeta - the alien shit is kinda cool and I like the weopons you end up with. if you want the achievement for getting all the alien hollotapes make sure you get them all before you finish the story because you wont be able to go back, unless you have another character to load and play it again
4. Broken Steel - is the one that puts your level cap at 30 instead of 20, which was nice
5. Operation Anchorage - was kinda lame I thought
like I said, just watch the videos online at IGN, youtube, etc. Or read about them on The Vault Fallout 3 wiki. I bought them all right when they came out but I probably have a shit ton of hours played on this game, with three characters to play as good evil and neutral. the game is just so deep and theres so much you can do im just trying to knock out the last couple achievements (data miner, 100 ingots, and all alien holotapes) so I can give up my Fallout addiction finally
