It's cheap, it's readily available and it does wonders for the garden.  

As an added bonus you can pause your gardening tasks and take a spoonful for extra energy!!!  :0)

In a nutshell, molasses is high in potassium, calcium, iron, magnesium, phosphorus and other important micro-nutrients - not to mention heaps of sugars which beneficial soil bacteria LOVE!  Adding molasses to the garden has been shown to benefit the bacterial health of the soil by feeding the bacteria these sugars.  In so doing, the plants (vegetables) become more more receptive to soil nutrients and fertiliser and, in general, become healthier and more resiliant to pests.  When applied to tomato plants it has an added affect of making the fruit sweeter.

The best way to apply molasses is by adding 1/4 to 1/3 cup blackstrap molasses to your 9 litre watering can and mixing it in.  Then a light water over your entire garden will do wonders.  If your soil is very poor 1/3 cup is recommended with the smaller amount useful as a general tonic.  Apply only once per month.

Interestingly enough, this method is also useful to treat root nematodes as well as being useful as a possum deterrant.  When added to the compost heap it provides extra nutrients and gives it an extra boost for use as a compost starter.

For more problematic areas a more concentrated molasses spray will do the job.  If you're experiencing trouble with caterpillars, aphids or mealy bugs, curly grubs etc. the following recipe should help.  

1 tablespoon Blackstrap Molasses
1 litre Water
1 teaspoon liquid soap
Place into spray bottle, allow to dissolve then spray over plants. 
TIP:  Don't leave in the bottle for too long as it will start to ferment and possibly EXPLODE!!!  :0(

Molasses with the addition of epsom salts makes a super tonic which will improve a plants roots and cause increased flowering (which means more fruit!).

2 tablespoons Molasses
1 tablespoon Epsom Salts
7.5 litres Water
Allow to dissolve then water over plants.  Use around once every 2 to 4 weeks.

Unsulphured blackstrap molasses is the best to use.  The sulphured molasses sometimes available can alter the pH of the soil and is not recommended.  

Molasses is readily available in supermarkets, however the cheapest source is from your local farm supplies store where they sell it by the Litre.