In sports anime, I guess the first season of Prince of Tennis had a lot of fillers. xD Some of them are unrelated to the main plot like "Prince of Bowling" and the "TeniPuri family". They're quite entertaining to watch, though... However, some continue from the main plot but are quite unnecessary. I skipped some of them when I watched it the second time. lol I just want to watch the matches soooooon!!!!
Anyway, these fillers are more common in shonen anime like Naruto... xD