Now it's getting to winter people start to realise how poor their lights are compared to the latest generation of cars. A popular upgrade is to fit xenon headlight bulbs which give a 'whiter' light. I am not talking about the blue bulbs that seem popular with the boy racers (see below), genuine xenon bulbs will improve the light output as well as apperance (to some extent) of your headlamps. Please note: No bulbs that I have come across will give the same appearance as HID (high intensity discharge) lamps as fitted to high end cars like mercedes,bmw and audi's - this is down to the construction of the headlight itself - if you buy bulbs expecting the same effect you will be dissapointed!
The bulbs will have a colour tempearture rating (xxxx K), and is a measure of the 'whiteness' of the light emitted - it has nothing to do with the operating tempearture of the bulb (ie how hot to the touch the bulb is). The lower the xxxx number is (say 3300K) the 'yellower' the light is, the higher the number (say 6000K) the 'bluer' the light will be, but the trade off is that the light output will appear to be less due to blue light being difficult to see by. Also because our eyes are more sensitive to the yellow end of the spectrum, a 50W blue bulb will appear dimmer than a 50W yellow bulb due to our eyes sensitivity. In wet conditions the water actually absorbs a lot of the blue light so the light output will appear even lower.
Note: The colour temperature of natural daylight at noon is between 5000-5500K.

So blue bulbs may look cool, but if you want to see where you are going and others to see you, avoid them like the plague!

So what am I using? Well I was running 90/100W standard halogen H4 bulbs (standard headlights should be 55/60W! ;-) ) but decided to change for Philips Vision Plus bulbs which seem to give the same amount of light output but with a better spread of light and a better colour (ie much 'whiter'); in our other car I have Ring Ultra Xenon bulbs which I am very impressed with (and would probably replace the Philips ones with when they go) despite the Philips bulbs getting a better review. I have also got a pair of Osram silverstar bulbs for my fathers car as they got the best award for H7 bulbs in one of the motoring magazines, but haven't had an feedback from him as yet.

Update: 6/11/07: There is a new bulb on the market, the Osram Night Breaker which is (according to the manufacturers) supposed to give 90% more light output and be 10% whiter (I assume measured against standard halogen headlights). I will be trying these when I next require replacement bulbs but they are not cheap (at the moment) with prices at approx £20 for a pair of bulbs.

Update 1/10/08: The Ultra Xenons never failed before I got rid of my car so I never got to try the Nightbreakers so I can't comment on there performance and bulb technology has now moved on again. The latest bulb to get rave reviews is the Philips X-Treme Power and I have a pair of these waiting to be fitted to my new car when it arrives, if they are better than the Vision Plus bulbs I will be impressed. Also just arrived on the scene are the Ring Xenon Max bulbs (so new they have not appeared in any reviews as yet!) and if they are anything like the Ultra Xenons by Ring, they should be excellent. I will probably try the X-Treme Power bulbs in the dipped beam and the Xenon Max bulbs in the main beam.

A couple of points to note are:
 a) Keep the headlight glass clean - you'll be amazed how a quick wipe will improve the light output no end
 b) If you change to xenon headlight bulbs you may want to change your sidelights too, but beware of cheap, no-make LED bulbs, their light output is generally low, not very white and tends to be in one direction only (although this situation is now improving).
 c) These bulbs seem to blow more often, probably due to the higher output and/or the construction of the bulb itself.