I was partly inspired by aes0ps post on the siege placement on veil (and used his example here), and I've been wanting to put sometime like this together for quite a while. Someone mentioned that LazyEye was also doing something like this, but I was mostly done already so I just went ahead and finished. I didn't really include turret placement\ perfect PG placement in all of these, and a lot of these spots havn't been tested in an actual game other than to see if they reach, so if somebody tries a spot and it fails horribly post here about it and mention what could be improved

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I've played many many NS games and seen many comms do lots of different placements of sieges, some good, some bad. But what I only see very few do, is take full advantage of the distance granted between the TF, the sieges, and the distance from the sieges to the hive. Another is positioning your sieges and TF so they allow for ample space and fighting room for your marines w/o tripping over everything, and their guns hitting siege hitboxes instead of the skulks and fades. Your marines will thank you.
The two main things you should keep in mind while viewing through all the examples:
1) The distance circle from the TF to the sieges: Turrent Factories have a fairly large radius that you can place sieges within. In most maps, this means that you can snuggle a TF out of the main battle area by the sieges and keep it a lot safer than trying to jam it in with the rest of the sieges.
2) The distance from the sieges to the hive a lot of times comm's are not sure if they will reach or not, so they put them somewhere where they KNOW it will reach no matter what. But I have gone through a lot of the commonly played maps and showing examples where you can place your sieges much further back from the hive and still hit.
I'm not saying that every single one of these is the "best" siege spot because that really depends on what your up against and the situation (ie.. bile bomb, fades, etc), but a lot of them should be helpful to gauge how much distance you have to play with at the least. One thing I should mention is that in each one of the examples, the sieges will ALL hit from those spots, although sometimes you might have to practice offline to be able to place them quickly since you won't have time in a game for trial and error. Keep in mind a lot (not all) of the shots I have, have to be placed in that spot exactly or closer, because they are in the absolute farthest away they can be and still reach. I just wanted to demonstrate how much space you really have, so again some trial and error yourself is helpful to get a feel for it.
To do so easily, type this in your console.
bind "x" "sv_cheats 1; bigdig; givepoints; spawnhive"
Then create your own server and whatever map you want to practice on, and hit f11 to turn cheats on, enable insta-build, give resources, and spawn hives. The spawnhive one is good if your practicing getting the siege spot right to hit the hive. The "x" can be any key you want.
Here are some of the nicer sieges I wanted to show:
NS_VEIL:
I've used this one before and it usually works better than having them in the other room as their more protected from bile bomb. Although you could also put that 3rd siege in line with the other 2 and it still hits and it frees up that side of the hallway for marines to move (I didn't feel like re-doing the picture). Seems to be a lot cleaner siege than in the other room IMO.
NS_TANITH
I like this one because you can snuggle that TF in that space where the aliens won't notice it, and it will be a hard target. Plus those sieges along there arn't visible from the waste hive side till you go deep in the hallway (better protection from bile bomb), and it gives your marines plenty of room to fight.
NS_NOTHING
This is a common spot where I always see comm's putting their sieges way at the end of the hallway where the lerks can easily gas them from the overhang on via and skulks\fades have a big open area to attack. When their in the hallway you can have your marines way up front fighting and one or two building the nice row of sieges from behind, and your TF is completely out of the way.
NS_LOST
Self-explantory, much cleaner than putting them in the other room.
NS_HERA
Much furthur away from the hive, and your marines have direct line of sight to the TF and sieges when they phase in.
NS_ECLIPSE
I really like this one. TF is out of easy access for aliens, and not in the way of rines. I don't think I've ever seen a comm put the sieges in that room instead of the one in front of it... very handy. Plus the turrets there really make it fun for the skulks.
NS_CAGED
Aliens can barely see them running by

and the whole hallway is wide open for your marines.
NS_ORIGIN
WAYYYYYYY back from where most comms put them. All reach of course, although might be a little tricky for marine footing, would require testing in a real game scenario.
Download:
Siegeguide.zip
(many thanks to paranoid for hosting the download)