Well there's The Webcomic List, Top Webcomics, and The Belfry Index. I don't know how much success I've had with any of those. There's not really as much of a community as here.
I used to use the forums at Ink Outbreak! I really liked it there and they had a cool banner ad exchange thing... but their website was wonky for a while and I stopped checking it.
Honestly, regular social media is where I get most of my new readers from. Twitter, Tumblr, Insta, Facebook. Only people who are super into webcomics are gonna be on sites specifically for webcomics, but you can appeal to a larger audience on social media.
Edit: I forgot, there's also Comic Rocket.
