If you're enjoying Curveball I'll admit I'm reluctant to let go of you. :) That said, if you're looking for other good serial superhero fiction on the web, it's certainly out there.
- We'll start with the elephant in the room: Worm, by Wildbow. Worm is probably the nine thousand pound gorilla of superhero webfiction. It's also a finished work, which means you won't be left hanging if you start it. It's also dark. Wildbow usually describes himself as primarily a horror writer (and it appears his latest project will tilt hard in that direction) and the world he creates is very, very grim. And frequently depressing. But it's also extremely well done.
- There's also The Legion of Nothing, by Jim Zoetewey. LoN tells the story of third-generation heroes who inherit the base of one of the famous superhero groups of the Golden Age. The story starts with all the protagonists in high school and so far they've progressed on to college. LoN was actually the web serial that convinced/inspired me to start Curveball, so you have Jim to blame or thank for it.
- There's Special People, by Tim Sevenhuysen. A wonderfully bizarre story where most people have powers that might be considered, um... trivial. Although I personally think the ability to conjure bacon out of thin air is probably one of the most fantastic powers ever created, ever, in the history of everything.
- Super, by Palladian. Focuses on a specific team of powered "troubleshooters" and the unemployed young woman who joins them.
- Wonder City Stories, by Jude McLaughlin. An unusual serial that spends a lot of time looking at the day to day lives of characters with super powers -- not just the part where you don the mask to fight evil, but also the day job, dealing with the kids, growing old, etc. This is a brief description and therefore insufficient to adequately explain it. :)
- Snake Boy Loves Sky Prince, by Joey Manley. Unfortunately we'll never get to see the end of this story, because Joey died in November. And this isn't even the most recent version of the serial--he was actively rewriting and re-plotting it. But this snippet is so wonderfully, anarchically funny I feel the need to include it in this list.