The 50mm 1.8 is a must-have. Just get it.
For a general walk-around, the 28-135 IS USM is a good choice, or if you want nice walkaround L-glass, the 24-70L is an awesome piece of work.
They also make some 28-105 glass, but I think you may be better off in the Sigma or Tokina arena there.
Others may have other good advice.
Concur on the 50mm. The 28-135 is ok - it's not the world's sharpest or fastest lens, but for the price, it's decent value.
I wouldn't bother with the 28-105. Much better to go with the 28-135. Or if you want that sort of range, the 24-105mm is L series glass, and is (from what I hear) comparable to the 24-70 ... if you'll be happy with f/4 rather than f/2.8, it's a bit cheaper, too.
If you're happy to buy EF-S glass (on the understanding that it won't go on a full frame or 1.3 crop body - ie, it only fits on the 300D, 350D, 400D, 20D, and 30D to date, with the possibility of more bodies in the future), the EF-S 17-85mm is also good value for money. The only reason I regret getting it is that I've decided I want to move to the 5D (or its successor) in the future, and I won't be able to use it on that body.
One image I took with the 50mm f/1.8 on a 20D:
Depth of field is a bit shallow in that shot, but that was because I don't like stopping that lens down - the five blades makes for weird bokeh. I want the f/1.4, but can't afford it for now.