There may be 100s of TV series that have more than eight seasons. You can check this IMDB page for the complete list. I have selected some of the top series for the answer.
Here is the list of 50 TV series with 8+ seasons:
- All in the Family
- Aqua Teen Hunger Force
- Backyard Farmer
- Columbo
- Cops
- Creative Living with Sheryl Borden
- Curb Your Enthusiasm
- Don Francisco Presenta
- Entertainers with Byron Allen
- Forensic Files
- Friends
- General Electric Theater
- Ghost Hunters
- Good Morning America
- Gunsmoke
- Hallmark Hall of Fame
- Happy Days
- Ice Road Truckers
- Illegal Knowledge
- Inside Edition
- Kickin' It with Byron Allen
- Let's Make a Deal
- Little House on the Prairie
- Live from Lincoln Center
- M*A*S*H
- MTV Unplugged
- My Classic Car
- Mystery Science Theater 3000
- Portlandia
- Professional Bowlers Tour
- Rick Steves' Europe
- Saturday Night Live
- Seinfeld
- Sesame Street
- Squidbillies
- Ted Mack and The Original Amateur Hour
- The Danny Thomas Show
- The Goldbergs
- The Jack Benny Program
- The Late Show
- The McLaughlin Group
- The Office
- The Simpsons
- The Walking Dead
- The Waltons
- The Wonderful World of Disney
- The X-Files
- Truth or Consequences
- Two and a Half Men
- Wild America