Better Move is a core Chisel plugin that allows you to actually move the entire map on the Forge grid. This is really useful when you run out of room on one side... Version 2.2 actually works on maps with annotations, unlike previous versions. It also adds several cool features, like the ability to move platforms, and the ability to run it more than once on a single map. This plugin is also available as part of the Chisel 2.1.6-enhanced package, and is part of the standard Chisel 2.1.6 package if you downloaded after December 6, 1999.

Monster Converter is a Chisel plugin that lets you avoid an unpleasant quirk in Forge. When M1 maps are converted to M2 or Infinity maps, Forge ignores the fact that the monster order changed... so you have to go through your maps and click every single one to convert it. (Or you have to nuke the map.) Monster Converter fixes this. This plugin is also available as part of the Chisel 2.1.6-enhanced package.

Poseidon is a Chisel plugin that lets you flip a marathon map (M2 or Infinity) around the X, Y, or Z axes. Pretty slick, actually. Version 1.1 fixes a number of small bugs. This plugin is also available as part of the Chisel 2.1.6-enhanced package.

Stretch is a cool Chisel plugin that allows you to stretch or mirror textures on walls. 3.0 is a pretty major upgrade that allows alterations of lines (on the automap), so that platforms can be hidden, and various other visual tricks can be performed. Well worth the download if you're building cool maps...