I know there are a few sports-comics here on Tapastic, though I don't read any of them myself - but if you're into professionally published manga, may I recommend Eyeshield 21 and Yowamushi Pedal?
They're both really, really good (And YowaPeda is, er, completely bonkers), and I may or may not have read the first 100 chapters of YowaPeda in a single day because oh my god.
... Whatever you do, do NOT watch the Eyeshield 21-anime. It does not do justice to the absolutely wonderful thing that is the manga.