It's better to call them Private Servers.
I know two good ones:
Worldscape, the awesome and spreading Private server with features such as:
"Special" Partyhats
Ancient Magicks
Over 99 leveling (ex. attack level 99, 100, etc...)(Max: 135)
Getting combat level as high as 172 (More for admins)
Ownage Teleport codes
Sounds and Music
and more........
Come play now at http://www.worldscapeblitz.com/
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Holypkingscape
The coolest P.K.ing Client around.Features include:
Lunar and Ancient Mage
Godswords
Special Attack
Master Code (::master)
Nice Staffs
Ownage
Mini-Games (like Barrows, Barbarian assault, etc...)
and more........
Come download now at http://holypkingscape.webs.com/
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Those are the ones i know best.
Try them out.