If you are looking for a lens to shoot wildlife and birds, I highly recommend the Canon EF 100-400mm f/4.5-5.6 IS telephoto zoom lens. This is a superb lens, decently fast, and provides a great focal range for capturing wildlife and birds on the perch. It is not quite fast enough at 400mm to really help you capture birds in flight, however with good lighting and hand-held technique it can serve that role as well. The optical clarity of this lens is excellent as well. It does have IS, which can provide up to 3 stops of slower shutter if you use a good hand-held technique, and that offsets the slower aperture a bit.
This particular lens uses a push-pull zoom, which some find a little odd. After using it for a bit, I have come to love that design for telephoto zoom lenses, as it allows much quicker adjustment of your focal length than a ring-style zoom. The 100-400mm does not rotate when focusing or zooming, so it is easy to use with filters.
When it comes to telephoto lenses, a key thing to understand is that even with 400-500mm worth of focal length, you still need to get fairly close to capture a good shot with nice bokeh. With wildlife, you can be up to a couple hundred feet and still get a decent shot. Frame-filling shots still usually require you to be within 50-100 feet of your subjects. With birds, you need to be within 10-15 feet usually to get a frame-filling shot, so the technique you use to sneak up on your subjects without scaring them away is critical.
The difference between 400-500mm is small, but in the case of birds, useful. You get maybe 20% tighter framing and a little more reach. Sigma makes a couple lenses that reach 500mm, one of which is the 150-500mm OS lens. This lens has a pretty slow aperture, with the maximum being f/5 at 150mm, and f/6.3 at 500mm. Even with the image stabilization (OS), a maximum apertyre of f/5 is not ideal for working with birds. For wildlife, with good hand-held technique, this lens should be a decent lens. At its price point, some $2400 (cheaper street, probably around $2000-$2200), I would stick with the Canon EF 100-400mm, which lists for $1800 (street price is around $1600.) In almost every way, the Canon 100-400 is a better lens.
Another alternative would be the Sigma 120-400mm OS lens. This is a very competitive lens, and compares very well with the Canon 100-400mm. It is a bit cheaper than the Canon, and uses rings for both focus and zoom (in case you don't like push-pull zoom.) It costs a bit less, as well, at around $1300.
EDIT:
Regarding filters, like a polarizer. You can indeed buy screw-on filters for your lenses. However, if you begin to invest in a wide variety of lenses (which is ultimately inevitable if you stick with photography), you should look into a flexible filtration system. Both Lee and Cokin make great multi-filter holder systems that use adapter rings to connect to just about any lens with filter sizes from about 40mm up to about 100mm. The quality and versatility of filters available for a filter system like this are much higher than your average screw-on filter, and given that they can be used with multiple lenses, despite their higher up-front cost, they are much cheaper in the long run.
I recommend Lee myself, as their quality is unsurpassed. They do have some supply problems, and it can be difficult to find their filters and kits at times. Cokin seems to be much more readily available, however the quality of their gear is a little lower. It is also possible to use third-party filters in both of these systems. An example would be the very high quality Singh Ray filters.
