The Connected Threshold Growing plugin can be used to perform segmentation of 2D and 3D images. The plugin accepts 8 bit and 16 bit gray images. To perform segmentation you specify location of seed point (x,y,z), and minimum and maximum limits on pixel intensity. Segmented region will contain all pixels connected to the seed point which intensity is within minimum/maximum intensity limits.
The example demonstrates segmentation of a 3D MRI image from BrainWeb Project. Here is slices 31 of an image used in the example:
The image was segmented using only 7 seed points (white matter, gray matter, skull, left eye, left lens, right eye, and right lens). White and gray matter regions were combined before 3D rendering using ImageJ image calculator. The 3D rendering was performed using VTK (SmoothRender.java)