I just bought a Canon T2i and I'm concerned that I need a UV filter (for protection) and I have some questions.
What are the best brands of filters?
What (side)effects filters brings to the table?
Other tips from you pros are very welcome!
A UV filter will provide protection for your lens at the expense of image quality.
Under normal circumstances a UV filter is not needed and better protection is afforded by using a lens hood. When the lens is in eminent danger from flying particles of sand or salt water, then it is time to use a UV filter since the lens hood will not protect against that.
Tests have been done with plenty of UV filters. Different brands also have different grades. The better ones within a brand are usually labelled 'Super Multi-Coated' or at least 'Multi Coated'.
Hoya makes the best filters I have used but, well, I have not tried all the brands but I stay away from B+W which are not color-neutral as a UV filter should be.
NOTE Some shops confuse UV and Skylight filters which add a pink cast to everything unless you use Automatic White-Balance computed by the sensor which will cancel out the cast.
TIP Save money on filters buy buying the largest size and step-up-rings to avoid needing one filter-per-lens. The catch is that you use either a filter or a lens hood but not both because the step-up ring gets in the way.
Lens-Tip has compared 20 different UV filters (results are here). Hoya is holding the first three positions, but it also has a model in the middle and another one near the bottom, so brand is not all you need to look at, different models really are different.