Further to some of the posts above:
The 'equivalent wattage' quoted on some bulbs need to be viewed with caution. For example, you might be seeking to replace a (tungsten) 40W bulb in a table lamp. Manufacturers like Philips will tell you that you require an 8W low-energy bulb for the same light output. That statement is perfectly true but if fails to take into account that the light from low-energy bulbs is in a slightly different part of the spectrum from that from tungsten bulbs. Consequently, the human retina can perceive the light-levels as being slightly lower than would be expected. The solution would be to 'go up to the next step'. i.e. purchase an 11W bulb (which is described as being equivalent to 50W), rather than an 8W one.
It should also be noted that the 'cheapo' low-energy bulbs can't be used with dimmers. You can buy dimmable bulbs but they do cost quite a lot more. (Several pounds, rather than pence).
Similarly, specialist high output bulbs (for people like Mamyalynne, with sight problems) are also available - but not cheap:
http://tinyurl.com/28m7u2u
Chris