I am looking for a lens that does well with
- backdrops
- low lighting
- out side
- action wise swimming in the pool
- sports
- some portraits outside
It sounds like you are looking for a large aperture lens (low f-stop) lens so that you can get good low-light and action shots. In order to get good low light shots without image stabilization, you generally need an f/2.8 lens or faster. You can use an f/3.5 lens with image stabilization (IS) in low-light, but the IS won't prevent blur from the subject.
Here is a fairly comprehensive list of Canon lenses... If you exclude all of the lenses with apertures slower than f/2.8, that leaves mostly L-lenses and prime lenses.
The cheapest way to get good low-light and action shots is with prime lenses. If you are willing to go without zoom, you will be able to get a much larger aperture (50mm @ f/1.4 or f/1.8, 100mm @ f/2.0, etc.) for the money. There are a lot of Canon prime lens options available for less than $1,000, ranging from 20mm to 135mm. I would say make sure you are comfortable with a fixed focal length before moving to prime lenses however. If you are happy with 50mm, the f/1.4 provides amazing low-light / action performance, and great sharpness when used at f/4. The 135mm f/2L is another amazing prime lens which would be perfect for outdoor sports photography, but it is right under $1,000, and would be a little too much telephoto for general purpose use.
If you must have zoom, there are still options for under $1,000. The 17-55mm f/2.8 IS USM is available under $1,000 used, and it offers a nice wide aperture combined with image stabilization. I personally think it is a little soft and has noticeable chromatic aberration, which for the price was unacceptable to me. I was picky about those things and purchased the 24-70mm f/2.8L USM instead. I wanted a no-compromises lens and with the exception of image stabilization, that was it. The 24-70mm has weather-sealing as well, which protects it from dust and water intrusion when used with a fully weather-sealed camera body (the 50D is not weather-sealed around the lens mount from what I have read). Unfortunately the lens is around $1150 used, so scratch that idea.
You may also want to consider a used
28-70mm f/2.8L USM, which is the predecessor to the 24-70mm.
Here is a comparison of the 24-70mm and 28-70mm together. The 28-70mm usually sells for $800-$900 on eBay. It does not include weather sealing but otherwise it is a great lens. Like the 24-70, it is heavy but worth it for the image quality.
As for non-Canon lenses, I have seen results from the Sigma 28mm f/1.8 prime, and it is good. I was initially tempted by the Tamron 18-270mm for its very long zoom range, until I saw bad image samples and heard that people were having issues with the zoom ring.
As someone else mentioned,
The Digital Picture is a great website for lens reviews. I recommend using their ISO 12233 comparison charts so you can see how various lenses perform against each other.
Good luck with your lens choice!