For many indoor growers, inexpensive and readily available fluorescent bulbs are a good option. Even a simple compact fluorescent bulb in a lamp fixture can provide enough light for a few plants, and takes very little space or electricity to run. However, for those with a little more space, four-foot fluorescent tubes in a shop light fixture will provide more light for the amount of electricity used than a compact fluorescent bulb.
If you do decide to use fluorescent bulbs, you may find the variety of bulbs overwhelming. Should you choose warm white, cool white, daylight or plant and aquarium bulbs? In the past, many gardeners have recommended mixing one cool white and one warm white bulb to provide a wide spectrum of light. Some people now claim that the newer "daylight" bulbs provide more of the colours of light that plants can use. The plant and aquarium bulbs are specially formulated to only produce the types of light that plants use for photosynthesis, so although they may appear dim to our eyes, they are using electricity more efficiently to produce only light the plants can use. Although some people report great success with these lights, they are also significantly more expensive than other types.
When choosing the colour of your bulbs, they usually have a "Kelvin" rating to indicate the colour. 3000K, for example, is usually considered warm white, while 4100K is cool white, and 5000K is sometimes called daylight and 6500K is sometimes called daylight. The higher the number is, the bluer the light will appear. It is often thought that plants use blue light for vegetative growth and red light for flowering, so seedlings probably mostly need blue light. However, I have had strawberry and tomato plants produce flowers under Philips Daylight Deluxe 6500K compact fluorescent bulbs, so even these bulbs may produce enough red spectrums for plants. Another option that some gardeners find gives success is to mix less expensive bulbs (cool white or daylight) with one or two of the more expensive grow lights to provide a more complete spectrum.
The next question to answer is: T12, T8, T5 or compacts? T12 bulbs are the older tubes that are 1.5" in diameter. They are the least efficient and the least expensive. The newer T8 bulbs, which are 1" in diameter, usually produce the same amount of light, but use less electricity to do so. These bulbs and their more efficient ballasts are now readily available and usually only slightly more expensive than T12s. Although they have a higher initial outlay, the cost savings on energy probably makes them worthwhile. They also tend to have a lower rate of "lumen depreciation", which means that their light output decreases less quickly. This means that they do not need to be replaced as frequently. Some sources suggest replacing fluorescent bulbs every season because, although they appear to be working fine, their actual light output decreases enough to cause problems for the plants growing under them. Many newer T8s make claims that their light output decreases by less than 10% over their lifetime, compared to about 25% for T12s. T5 bulbs are even smaller and more efficient than T8s, but they are also still somewhat expensive and difficult to find. They also run more efficiently at higher temperatures than T8s do (about 35C rather than 25C), which could be good or bad depending on how you use them. Compact fluorescents are generally somewhat less efficient than the tube-style lights, but they are readily available, inexpensive and will screw into any regular light fixture. They can be useful for spotlighting or using in small spaces.
In conclusion, the most cost-effective solution (right now) is probably to buy shoplight fixtures that will run T8 bulbs. Then purchase whatever bulbs you have a good feeling about, and if you have enough of them, you will probably be able to successfully grow some plants under them. I recently purchased a case of Sylvania Daylight 6500K T8 bulbs for what worked out to about $3.30 (CDN) per bulb to use with my new grow shelves. If I want my plants to flower and I don't have success with these bulbs, I'll probably try mixing in one grow light per shelf.