For new witches on a budget, or for those still in the "Broom Closet" for whatever reason, let me share one of my best and simplest altar tools: the tea-light oil diffuser / warmer. 

You can find these very easily at dollar stores, Walmarts, and other places such as thrift stores quite easily and they are relatively inexpensive, from between $1-$5. Some people make their own and this is a wonderful practice, so long as the vessel is indeed heat-resistant. The oil diffuser, shown below is a small soapstone one, they also come in metal, ceramic, and glass. Choose one that best fits your individual taste and budget; I like the soapstone variety, like the one shown above, as they have a more natural feel to me.

How do they work as an altar tool? There are a few things this simple tool can represent. The first is the elements: Fire from the tea light, Water from the liquid used to fill the hollow, Earth from the container itself and from the herbal qualities in the essential oils used, and Air in the evaporation. Secondly if we view the fire within as an Active and the water/oil mix as a Passive, we can see balance or even Masculine / Feminine, perhaps even God and Goddess energy. If viewed as a Seed [fire] within the hollow belly of the Mother [earth, the container], we can see Fertility, even Motherhood. Depending on the practitioner it could represent many other things; the limit is your own mind, after all. 

The diffuser can also be used in spellwork application. If you wished to cast a protection spell you could fill the bowl with water and some drops of an oil from a plant associated with protective qualities, such as Rosemary, and activate it by lighting the tealight. Need to work on Self-Love? Try a rose oil, Etc. Intention and activation to send your spell out to do its work.

Now some people will note that not all essential oils are made equal. A lot of the ones you can buy easily and cheaply are not the best quality, or are blends, and some are even synthetic. Does this matter? Personally if I had to use essential oils on or in my own body I would want to get the better quality ones. Otherwise I have no problem using the others in this sort of work. It is my opinion that there is no magic or devotion that is worth hindering yourself financially, and that there is no object that is inherently sacred until you make it so. If you want to make the act of setting up and using a diffuser like this into a sacred act, make magic with it, or a devotional offering... then you can. 

Some Pros: Innocuousness: a single small area with a diffuser can be an entire working or devotional space without other people knowing what it is. A diffuser lacks the smoke of incense with can set of smoke detectors or cause issues with people who have breathing problems. The lingering scent of the evaporating oil can help focus the mind on the intention being sent out. They are inexpensive and usually easy to find. You can use scented wax with them. They are highly portable. 

Some Cons: Use active fire and must be tended like any other candle. Often has synthetic or blended essential oils. Usually you have to refill the water/oil blend before the tea light is out, or the tea light before the blend is evaporated, depending on the size of the bowl. Have to buy tealights; while they are inexpensive it is a continual upkeep. Have to buy oils; while they can be inexpensive it is a continual upkeep. If you use wax in them you have to clean them often. If water is left to evaporate in metal or stone diffusers can cause rust or erosion if the diffuser is low-quality. 

Electric diffusers are available at a slightly higher cost. Wax warmers can be used in a similar fashion but both of these lack the genuine element of fire; however if you cannot have open flame in your home, it is a viable alternative so long as caution is used regarding water + electricity. 

What if you do not want to buy one? What if you already have a vessel that you can use, such as a small cauldron or footed bowl? As long as it is heat-resistant and you cant watch it, why not? Keep an eye out for material that is porous though, for they will not hold water / oil easily!

For the truly crafty witch, you can use your own home-made essential oils should you wish. You can make devotional blends, spellwork blends, and more. The possibilities are manifold.