If the 24-70 f2.8L is too pricey for you, look into the 28-135 f3.5-4.5 IS
USM.  It has image stabilization, with which you're familiar, is lighter
than the 24-70, gives very good images, and is under $500 at B&H, among
others.  I have both lenses, and use both, since the IS gives me a 1 stop
advantage, hand held, over the "L" at the long end, and has more reach.
Failing that, the Canon 28-105 f3.5-4.5 (NOT the 4.5-5.6) is a very good
lens for its price, and Tamron, Sigma and Tokina make 24- or 28-70 or 80mm
f2.8 lenses in the $600 range.  I don't know about the Tamron or Sigma, but
I have the Tokina 28-70 f2.8 ATX Pro II, an older version, and its image
quality is about on a par with the Canon 28-135 IS.

28-135 f/3.5-5.6 IS USM. I have had one of these for many years; it has
performed flawlessly and given excellent results, and is an excellent
partner for my 17-35 f/2.8L. People have criticised it for the slightly
"loose" feel of the inner barrel, but it does not seem to affect the
performance on mine.

David

I'll second Skip's recommendation on the 28-135 f3.5-4.5 IS USM.  This was
the first lens I bought and is now my everyday walk around lens.

Although I don't have L glass at the wider end, from experience with my
18-55 and 75-300, I suspect that with the overlap this lens has on your
other lenses you'll find less need to swap lenses while you're shooting.

My wife has the 28-90 Kit lens that came with her original film Rebel 2K
(she now has an Elan 7NE) and the 75-300 IS and she still tends to "borrow"
my 28-135 when she's off by herself.  I guess I'm going to have to buy
another one just for her.

--

Rob

My question is:  Are you trying to fill the gap between 40 and 75,
just because there is a gap or that focal length gap is where you NEED to
shoot?  If the first, a few words of advice:  You don't need to cover
every millimeter of focal length to take good pictures.  A VERY common
mistake among tyro shooters.  I've been shooting professionally for 30
years. For my 35mm film system, I only own 5 prime lenses -- 24, 35, 50,
85, 180 -- and a 2x extender. Even with all those gaps, I can shoot 99% of
everything I've been called on to shoot over the years. So, don't fall
into the trap of thinking you need to cover every millimeter.  In fact,
gaps can be cost effective and you loose nothing.  If you're not shooting
with those millimeters, why buy them?

Try this:  Keep track for a couples weeks or month of what focal lengths
you use and how often each is used.  I mean, write it down.  Also, keep
track in that 40 to 75 range, when you would like to, but can't -- no lens
-- shoot there. Most likely you will find focal lengths grouping around a
particular one or two or, maybe, three focal lengths. The focal length
that you use the most is your personal "normal" focal length. Try to get a
zoom where that focal length is at the center of the range, more or less.
This is the lens you'll use the most. As an example, with my 35mm film
work, my "normal" is a 35. I rarely needed anything wider than 24. Most of
my telephoto work is with the 85. I almost never used the 50. So, when I
wanted a zoom to fit on an old body for just snapshots and casual
photography, I looked for a 24-85 lens, but none existed.  I finally
settled on a fairly compact, not-much-bigger-than-my-50 24-70. So, when I
got my Canon D30 to have a similar range, I got an 18-50.  I would have
preferred something a little wider like a 15 or 16. The 17-40 was
not long enough, and I didn't want to buy a $1000 saddle for a $100
horse. ;-)

Now, if the reason to fill your zoom gap is the second option -- you need
to shoot with those focal lengths, have you thought of just getting a
prime in the 50 to 60 range?  Maybe, a macro?

Stefan