Hi, one piece of advice if you've spent a fortune on a very good camera like the canon 20d then it doesn't make sense to skimp on a polariser!!!
I'd go with what the other guys have said a circular polariser is the best bet, the main advantage being that you can rotate the polariser to change the effect. For example rotation the polariser can add some definition to clouds in a pretty uninspiring sky, totally cut out reflections on water or keep them as you wish. They can deepen the colour of skies and shiny objects too and seem to help with saturation of colours!
I generally use a skylight filter on my lenses if i'm not using a polariser partly because they are relatively inexpensive and a good way to protect a lens from scratches i.e. it cheaper to replace a filter than a scratched lens and also the benefit of eliminating a blue haze on your photo's that can be caused by uv light/shooting at high altitude.
I do this with a canon t90 which although it's an excellent camera is worth a fraction of the cost of your camera. If i'd spent a small fortune on the 20d i would invest in these kinds of things without fail, even if it's just to protect your lenses!
In terms of a brand I can only go from experience and i use a Hoya HMC multi coated circular polariser and Hoya Skylight/uv filter when i don't need the polariser. They seem to work perfectly well and i'm pleased with the results i've had.
I also learnt to be careful of the types of filters you use with a wideangle lens, i had several films developed where i'd been lazy and used the skylight & polariser on my lens, or used a cokin p series graduated tobacco filter and had some nasty vignetting around the edges of some pics! A costly mistake that ruined hours of hard hiking to get to the location i was looking for and hours wasted composing shots!!! incidentally on the same trip i missed a few rain drops on the lens and ruined some of the other shots too...Doh!
I'm pretty new to this so i learn a little more with each film i get back, that's the beauty of our hobby/passion it's a continuous learning curve that's get more rewarding the more you pick up!
Hope this helps
Laudrup