Some versions of the Adblock Plus extension work perfectly in Conkeror.
Adblock Plus 1.3.10 works perfectly with versions of xulrunner up through 27. It is broken in newer releases, so you will have to use a 2.x version in newer releases.
There are undoubtedly other good combinations. If you have one, please add the information here.
A complete set of versions of Adblock Plus is available at: https://addons.mozilla.org/en-US/firefox/addon/adblock-plus/versions/ You may need to save the .xpi file for your desired version; the .xpi files are linked from that page behind the "Download Now" button.
After installing Adblock Plus, you need to load the adblockplus.js conkeror contrib file (found in the <conkeror>/modules/extensions directory) You can then access the preferences dialog via (M-x adblockplus-settings) and load a filter set by selecting Filters->Add Filter Subscription and selecting one of the preloaded filter sets. You can potentially also work with filter subscriptions via the filters dialog (M-x adblockplus-filters) and add Adblock Plus patterns via (M-x adblockplus-add).
You may be able to access the Preferences dialog via the extension manager (M-x extensions), however see the extension manager page for discussion of reported problems with it not working correctly in some configurations.
Alternate instructions which may still be applicable in some cases
With the current versions of Conkeror and AdblockPlus, accessing filters via the Extensions dialog doesn't work because clicking on "Filter preferences" has no effect. You can work around this bug either by using the above mothod, or by setting the preferences in Firefox and copying them from the Firefox profile to the Conkeror profile. What needs to be transferred is the contents of the directory "adblockplus", whose exact location depends on your OS and perhaps the versions of Firefox/XULRunner you use. You can copy over all files in that directory to the corresponding directory in the Conkeror profile. You can also delete the one for Conkeror and replace it by a soft link to the Firefox version, in order to have all changes you make in Firefox propagate to Conkeror automatically.