PhpStorm

https://www.jetbrains.com/phpstorm/

Built in support for LESS.

Eclipse

Follow instructions here: http://www.normalesup.org/~simonet/soft/ow/eclipse-less.fr.html

When using this plugin, use @import without the url() notation.
This is still valid:

@import '../imports/less_demo.clearfix.css.less';

This allows the LESS plugin for Eclipse to parse @import'ed files and provide autocompletion of mixins and CTRL+Click to navigate of the definition of a mixin.