For those of you with more than one lens, what one could you not live without??

I only have one and it is the kit lens that came with my rebel XT, I want to buy another one for now and am having a hard time deciding which to buy. I know it depends on what you photograph but overall, what is your favorite?

My Canon 70-200 4L,the reason is the amazing quality of this lens.In my opinion there isn't any other lens with the same quality like as a L lens.
I regret that i got the canon 28-135 IS ,it supposed that i had this lens all the time on the camera.If i have to decide again,i would take the Canon 17-40 4L i had tryed it and is a wonderful lens
http://www.canon-europe.com/For_Home/Product_Finder/Cameras/EF_Lenses/Zoom_Lenses/EF_17-40mm_f-4L_USM/index.asp?ComponentID=108350&SourcePageID=26341#1

I do not have a favorite lens as all of them (6) are very good depending on conditions.
If I were you, I would get the Sigma 18-200. It is my most used lens as it covers a great range and it is light too. It is a well priced lens :)
Below are some photos taken with the Rebel XT and the Sigma 18-200
Some taken at wide angle or full zoom:
http://www.photoboard.eu/photography/showphoto.php?photo=914
http://www.photoboard.eu/photography/showphoto.php?photo=1087
http://www.photoboard.eu/photography/showphoto.php?photo=1425
http://www.photoboard.eu/photography/showphoto.php?photo=1419
http://www.photoboard.eu/photography/showphoto.php?photo=1421
http://www.photoboard.eu/photography/showphoto.php?photo=1824
Some action photos:
http://www.boxerworld.com/photo/showphoto.php/photo/59499
http://www.boxerworld.com/photo/showphoto.php/photo/59500

As you already have a 18-55mm, you might want to get a 55-200mm so you can cover the 18-200mm range but I still recommend the Sigma 18-200 because everytime you change lenses there is a risk of some dust going to the sensor. I have to say the Rebel XT gets dust easily compared to the D70 and those dedicated special cotton buds for sensor are incredibly expensive (I have already spent about US$150 for cleaning the sensors).

Another lens I like is the 50mm f1.8. It is a cheap lens (US$80?) but very sharp and fast. Great at low light.
http://www.boxerworld.com/photo/showphoto.php/photo/60433
http://www.photoboard.eu/photography/showphoto.php?photo=1891
http://www.photoboard.eu/photography/showphoto.php?photo=1889

In my opinion there isn't any other lens with the same quality like as a L lens.
In my opinion too, but after the Nikon Pro lenses :D LOL

In my opinion too, but after the Nikon Pro lenses LOL

Yes,but TossBranAbi has a Canon Rebel camera ;) :D

True and it was probably the best choice for a digital SLR camera body at that price :)
I still think Sigma offers the best value for money. For the price of your 70-200 F4 (£478) I can almost get a Sigma 100-300 F4 EX IF (£549) which is a very well rated lens too. Actually I believe only that Canon 70-200 F4 offers a good value for the price. The other L lenses are great, but very expensive.

. Actually I believe only that Canon 70-200 F4 offers a good value for the price. The other L lenses are great, but very expensive.

Indeed!!Canon 70-200 4L is something like a bait,the lens with which Canon attracts us :p
I chose this not just for the price but for the light weight too.I couldn't hand-hold a heavier lens

I have four lenses but the one I used the most and does everything to a certain point is my Tamron IF28-300mm.

I've got 6 lenses at the moment, but looking to down size as I don't use a couple enough to warrant keeping them and it makes the bag too heavy!

For general purpose shots, I use a Canon 24-105mm f/4L IS. I've also got a 17-40mm f/4L for those super wide landscape shots but I'm going to sell it. As I have a full frame DSLR now 24mm is enough for me.

I bought a 135mm f/2L prime lens a couple of months, but found it to be too long for what I needed so am getting shot of that.

My recent (extravagant) purchase was the 85mm f/1.2L. I love this lens to bits. I am not a portrait photographer, which this lens is aimed at, but I love the shallow DOF it gives. This spends most of its time on the camera at the moment.

I've also got a 100-400mm, again, I'm looking to move this on in favour of a 70-200mm f/2.8. The 70-200mm is faster and has an internal zoom. I don't really need 200-400mm, and it is too long when at full zoom.

Lastly, I've got an 8mm circular fisheye. It's a bit of a niche lens, but gives some interesting results when the images are cropped. I've found you can get some really weird results using a flash as well.