Canon digital rebel XSI or EOS 450D is the best selling digital SLR at amazon.  There are two kinds of popular bags.  One is backpack to hold Canon XSi,  lenses, flash and other accessories.  The other one is top loading bag to hold the camera with standard lens only, such as the kit lens.

The best Canon digital SLR backpack is Canon Deluxe backpack 200EG-black.  This is the one I used over the last 5 years.  It was built in high quality nylon and very durable.  It can hold one or two  digital SLRs, 4 lenses, external flash.  There are many pockets to hold accessories such as memory card, filters etc.  There is even a front webbing ideal for lashing light jacket, sweater, or rain coat.  With the backpack, both hands and shoulder are free fro taking photos.  The only disadvantage is that it is quite hot when you carry the backpack on your back in a hot summer day.  This is something you need to consider if you mainly use it in a hot summer day outside.

The best top loading bag is Lowepro TLZ15 camera bag.  This is a rugged bag that  can protect both SLR and lens from the outside elements. The interior features a memory card pocket for quick changes and a brushed tricot lining protecting LCD screens and lenses from abrasion. The exterior of the bag is built with a tough water and abrasion resistant fabric and can be attached through either a belt-loop or with an included shoulder strap. This lightweight and compact, top loading bag provides digital SLR users with solid protection and fast access.  The main advantage of the top loading bag is that you can take the camera out quick.