After doing a bunch of research, I ended up with a Canon EF 50mm f/1.8 II lens. Complete opposite end of the spectrum from the various L series bits of glass linked above. Instead of dropping $1,500 or even $5,000, it cost me all of $76.30.
Why such a cheap lens?
Because it is a total gem.
The build quality is crap, but the image quality is outright amazing — doubly so for less than $100.
It is an all plastic lens with a focus motor that sounds akin to an old 110 film camera with an autowinder. Yet, that also means the lens is extremely lightweight. Combined with my Rebel XT body, it makes for one very small picture taking machine. It is probably 1/3rd of the weight of my 100mm Macro lens (that was pretty much my default lens until now).
With a maximum aperture of f/1.8, the lens eats light. Better yet, I’m finding that I can use the ISO 200 and ISO 400 settings of the camera without worrying too much about noise.
For example, the picture at right was one that I could not have taken with my other two lenses (the 100mm and a 17-85mm IS EF-S lens that is effectively a kit lens). I was shooting under a fairly dense pine tree in a relatively dark environment (all things considered). This is at ISO400 so could capture the always-in-motion nature of a 7 year old in a tree at a fairly high shutter speed (1/200th at f/4). It isn’t the best picture in the world — legs are a bit washed out — but I’m just happy I could take it at all and much more so that I could take it without really having to work at it much.