Unfortunately there are many Iguanas in rescue centres at this present time and not enough people who are willing to take them, this situation should ease off in the UK where they are very seldom imported now and hardly anyone is breeding Iguanas, but the situation is also bad in the USA with no sign of the many saturated rescue centres easing up in the foreseeable future.

If you currently own an Iguana and you are having problems, then first of all see if it is possible to solve them yourself with the help of the many various Organisations and forums available on the Internet, without actually resorting to re-homing, as this would not only help the burden on rescue centres, but also give you some satisfaction of achievement.
 
The two main areas which seem to cause problems are aggression and size,
(even worse when the two are combined)

Size,
If everyone did their homework on eventual size before purchasing, then there should not be any surprise....it's well documented on sites and in books today that you will end up with a 5 or 6 foot long lizard and should have been aware of the habitats and space eventually required, where space is limiting the habitat design, then it would be beneficial to the Iguana to be able to exit the habitat daily for a walk or exploration, this would not only help with exercise but also help with the Iguana's frame of mind.

Aggression,
In the vast majority of cases where solitary Iguanas are concerned aggression is only shown during breeding season, if your Iguana has recently started to act aggressively, gained orange colouration, tried to nip arms or legs and perhaps gone off food or is acting strangely in some other way (mostly during the winter months) then the good news is that this is almost always a temporary behavioural problem and your usually calm gentle natured Iguana will be back to its normal self in a matter of a few weeks.
Other causes of odd or aggressive behaviour can sometimes be down to bright clothes or even the type of perfume you are wearing! a new pet like a cat or dog bothering the Iguana, has he been teased or poked or mistreated by anyone without you knowing...in these cases resolving the irritation can usually retrieve the good behaviour.

If you have tried everything possible to help, or this behaviour has gone on for a very long time, or maybe the person who owned the Iguana has left for some reason and couldn't take it with them and no other member of the family or friends can help out....then, and only then, when all other options are exhausted, turn to the re-homing & rescue organisations.

Rescue and re-homing help in the USA..

Melissa's rescue page
Here you will find a very comprehensive listing of all the Rescue, Re-homing & General Herp societies in the USA, listed state by state...scroll down the page for the listings.

Green Iguana Society Rescue
Excellent pages on help by the Green Iguana society.

CORR.
Colorado Reptile rescue centre

Scales and Tails
Based in NJ but with helpers in various States, Gail's rescue organisation.

Triangle Iguana Rescue
Triangle Rescue org of North Carolina.

Mid Michigan Rescue
Rescue org serving the Mid Michigan area.

Wild green yonder
W.iG.Y. Indiana & Ohio rescue resource.

All species reptile rescue
All species reptile rescue based in New Jersey.


Rescue and re-homing help in the U.K.


The UK's Re-Homing Database run by the Ark Reptile Group


Simons Reptile rescue
A new Reptile rescue based in Malvern

Second Chance Reptile Rescue.
Tammies Rescue for Southern England.

Noah's Ark.
Sussex based Reptile and Animal sanctuary.

UK Animal rescuers.
The Reptile division of UK Animal Rescuers.

ASRA South Wales
ASRA South Wales Rescue Pages