Because GriefPrevention was updated in a major way in version 8.0, these extension plugins may not work with the latest GriefPrevention builds.
GreifPrevention-Anti-Worldedit disallows edits with the worldedit plugin when the editor doesn't have GP's permission to make changes in an area.
There is a DynMap plugin for GriefPrevention, which adds markers for player claims to your DynMap.
If you want to your players to buy/sell/lease claims, then you can install the Real Estate plugin for GriefPrevention. This Plugin has been updated by SuperPyroManiac to work with more recent GriefPrevention Builds. You can find his version of GP-RealEstate here.
GriefPrevention Flags adds flags to your grief prevention claims for PvP (admin claims only), and Mob spawning at the moment. (More features in the works!)
Flags Is a fork of GP-Flags that also supports other Region/claim type plugins, as well as adding several other features, such as support for Subdivisions, Multiple Worlds, and even flags on a world itself.
HardcoreClaims is a plugin that will allow a player's claims and GP information to be wiped when they die. thus every time a player dies they are set back to square one. It also integrates with Flags to allow per-claim control over whether a claim is deleted.