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The latest 18-55 IS is a good choice, the new 40mm pancake looks great as well and it's only a few hundred.
As Ian mentioned the 24-85 is a sweet little gem, no IS but size wise it is in between the 50 f1.4 and 85 f1.8, it's half the weight of your 24-105, has a 3.5-4.5 variable aperture which isn't much different then the 24-105's fixed f4 is.
I bought mine for 300 quite a while back, in 98, with one of my EOS-3 film bodies and it pretty much doesn't leave that body. I've a 40 and 50D that it works great on, when shooting with it I usually remove the grip and the combo is much lighter, fun to carry about.
If you don't need IS the 24-85 is a super 'beater' lens, one that has good IQ, but also I don't worry about too much given it's price point. Check out the reviews here on FM, it has it's issues, some distortion; at both ends, some softness, but it's fun to use. And with the 50D you've MA to fix any back focus issues...I highly recommend one.