Here's a league that I'm in...we're just starting off our second season. Contact Budstud if you're interested in being a fill-in down the road:
ASFL Season 2
This is another one I was in, but can't be anymore due to time constraints.
ESFL v.3 (3rd season just starting)
Contact 'jamesthecarman' if you're interested in joining that league. It looks like Cleveland may be open.
Here's Season 1 of the ESFL:
ESFL v.1
They're all run very well. The ASFL is just cruising along and has a LOT of good guys, as does the ESFL. Hope I could be of some help.
P.S. Both leagues have some SERIOUS rules that need to be abided by. Check the Rules link in the bottom right section of the site for league-specific rules (ex: defensive control, fill-in rules, etc.)