I don't really do the "Seasonal" beer thing. But I often enjoy certain types of beer at certain times of year, if that makes any sense.
Summer: I like lagers and good pilsners in the Summer months. Nothing better than a good New Glarus Edel Pils (made every other summer in New Glarus, Wi.) or a Sam Adams Boston Lager (my "default" beer) or a Stella Artois while watching baseball.
In the autumn and winter, I prefer darker, heavier beers and/or wheat beers. More calories = more heat. Yea! for heat. SA Boston Ale, Hacker-Pschorr, Hefeweises, New Glarus' Spotted Cow. Good porters for the win!
IPA's when I feel like it: Dogfish Head's 60 & 90 minute, Sierra Nevada, etc.