Hi Mark,
I have a 400D and use an old EF 75-300mm F4-5.6 Canon lens which I had with my old EOS 1000F film camera. I have found that this lens coupled with the standard 18-55mm has covered 95 percent of my requirements. It is auto focus but isn't the USM type which makes it a bit cheaper second-hand. It also has a 58mm thread for filters etc which is the same size as the standard meaning you don't need to buy more than one set of accessories to fit.
It is quite compact and fits in my small camera bag quite easily but on the down side it does suffer a little in poor light conditions.
The only other one I would consider buying would be a wide angle