Cleaning a fridge is not as simple a chore as it sounds, especially if it has involved a spillage of some kind.
We get a lot of calls over the course of the year that there is a reported fault of "bad smell from fridge", before I go any further let me point out that there are no components, mechanical or gaseous in a fridge that smell as a general rule.
In virtually every single case the smell is from a foreign substance in the appliance, usually spillage, that will cause this to happen.
The first are to check in a self defrosting fridge (not an air driven frost-free) is the drain channel at the back. Typically this is known as a "wet-wall" fridge, the moisture formed by the condensation proccess gathers on the back wall or, in older models, on the evaporator plate also located on the back wall of the appliance. This moisture forms small droplets of ice as the fridge cools and turns back to liquid form as the fridge enters a defrost cycle. Obviously that water has to go somewhere!
In almost all appliances of this type there is a drain hole below that wall, or plate, that the water runs into this goes to a tube that deposits the water in a tray on the compressor (motor). The heat from the motor evaporates the liquid into the air harmlessly. Often that drain hole can get blocked and this can lead to water in the fridge another fault caused by poor cleaning, check that first if you do have a problem as it may well not be a fault covered by warranty. If that drain is blocked, or partially blocked with spilled foodstuff then it can create a terrible smell in the fridge as the foodstuffs decompose as well as presenting a potential health risk. I can also assure you that Ragu sauce or something of that ilk that has been there for a month or two is pretty rancid!
The next are to look at is the door seals. But not on the surface of them, if you gently pull the seal away from the door you will see a cavity where spilled (in particular, liquids like milk) foodstuffs can be trapped, again leading to a bad smell as they decompose and, again, a potential health risk.
Shelf, particularly door shelves, should be removed regularly and thoroughly cleaned with proper detergents as this can also lead to odour problems.
A good thing to clean the inside of a fridge with is a lemon based agent, or lemon juice itself as being acidic it does cut through most grease etc. whilst leaving a pleasant odour. If you have had an odour problem leaving a lemon, cut in half in the fridge wil help remove any lingering smells.
Of course there are more effective ways of cleaning out the fridge and keeping smells at bay by using commercial products such as we sell in our store. For fridges you can see cleaning products from this link and, to keep smells at bay we find that both our Igloo fridge de-odouriser and the fridge mat do work well in this respect.
But recently we've also seen the introduction of the Kuro Cube, a neat little cube of charcoal that absorbs the smells in your fridge for up to a year meaning that you can have the odour eating ability of large American fridge freezers with their active carbon or charcoal filters without the cost. You can buy a Kuro Cube from this link
As has been pointed out since I forgot, you can also check the tray that collects water on the top of the compressor at the back (motor, black bottle type thing with the pipes coming out of it) as foodstufs can work its way down there and create an awful smell. Calcium build up can also occur there, again leading to a bad smell. (Thanks to Tinhips for the reminder)