Gadgets

Exterior gadgets are delivered by the factory. Since there is only one delivery man, this may take some time. Be sure to have a subway site directly outside the factory. Interior gadgets are collected by the tenant and thus cannot be installed previous to renting, but you can get the tenants to collect the first item before they even enter the house, preventing e.g. a noise complaint. Interior gadgets can cause the following complaint:

This stuff you just delivered is crap! It's about time you built a new factory otherwise nobody around here will take your goods.

You are actually supposed to blow up your gadget factory and build a new one. You can circumvent this complaint, like all the others, by evicting the tenant or selling the house, but since it is really connected to the age of the factory, it is better to comply.

Garden Shed

Required by the professor. Other tenants are unimpressed, but they want one if they see it in their neighbors' garden.

Garden Gnome

This hideous monstrosity will increase the happiness of your level 1 and 2 tenants by 10%. Higher level tenants are unimpressed, the Major detests it. He will file a complaint even if it is not in his, but in his neighbors' garden. You can hide the gnome nearly completely behind a deciduous tree, but you have to plant the tree first.

Garden Pond

You have to put one in the park the council demands after you have built all three brick houses. Otherwise, it seems to have no use.

Garden Furniture

When Yuppies and Sloanes demand "garden features", that's what they want. Required in the third council mission.

Home Repair Kit

Saves the repairman. Is used up over time. You can install a new one before the old one is completely used up so as not to run any risk. The home repair kit can only be installed in level 4 and 5 buildings.

Wishing Well

I have no idea what it is supposed to do.

Rose Arbour

You only really need this for the third council mission.

Greenhouse

Required in the second council mission.

Subway Site

Works as a teleporter. You can place a subway site on any estate owned by someone (you, an opponent, the council), but you can have only one per estate. The council has a subway site on the estate where its HQ is located, on the estate where your HQ is located, and it will put one on every third estate you buy. There's no telling on which corner it will place these sites, so wait till the sidewalk is fully developed before you place your own. Subway sites placed by the council are neutral and can be used by members of any team. Undesirables cannot use the subway.

Dog Kennel

I suppose this gadget is a bit buggy. First, the dog attacks only undesirables, it does not attack foremen trying to take over the building. I don't believe this is really intended behavior. On the other hand, the dog attacks the gadget factory delivery man! Since, finally, installing one can cause a tenant complaint which can only be solved by having a foreman blow it up again, I think it's best to leave well enough alone.

Trees

Required by all tenants except Punks, Nerds and Stockbrokers. Two will do, Slobs are even content with one. Whether or not they demand them, the first tree will increase tenant happiness by 10%.

Mouse Trap

Prevents infestation. Since it is an interior gadget, it must be collected by the tenants and thus cannot be installed in advance.

Computer

Allows tenants to breed higher-level tenants. Once a school is built, it becomes useless.

Double Glazing

Required by tenants living on the same block with a factory (resources only).

Burglar Alarm

Deters the thief and is thus useless when not playing against an opponent.

Window Shutters

Also called storm shutters, they are requested by the major along with double glazing if his house shares the block with a factory. Reduces the damage done by exploding buildings in the neighborhood.

It should be noted that lots of garden features is not a way to keep your tenants happy. The only gadgets that increase happiness are trees and garden gnomes, the latter for level 1 and 2 tenants only. Sloanes and Yuppies tend to demand garden features, but that is just a way to vent steam when they are unhappy (lower level tenants demand a lower rent instead). The main key to tenant happiness is the size of the garden. Upgrading the rooms helps too.

So what are most of these gadgets actually good for? Of course some may have yet undiscovered functions, but in general I think they were included more for the simulation side of things. Unfortunately most of them are completely out of scale, so they're not much good for that either.