It defaults to converting lava to obsidian, if right clicked on a block within 6 blocks of the player, and then searching a 5 block radius of that block converting all lava (flowing or source/still) into obsidian blocks (more or less as if doused with water (yeah, I know, flowing doesn't convert to obsidian)). Lava is NOT a clickable block itself (due to the nature of code I "borrowed" for finding the target, only solid blocks count, so target will be on far side of lava if clicking lava itself).
The target block and the replacement block can be configured via the config file (to, for example, convert sand to glass, or whatever).
The distance from player for a right click to "count" can be adjusted.
The search radius from target block can also be adjusted.
To install, extract the files from the ZIP file, place the JAR file into the Mods folder (in your .minecraft folder), and place the CFG file in the config folder (or skip that and it will create one with default settings).