IMPORTANT NOTICE - EMAN1 is now quite out of date, and is no longer being updated in any way. Modern standards require "gold standard" resolution assessment and many reviewers will also require things like tilt validation. All of these things are fully supported in EMAN2. EMAN2.1 has eliminated all of the issues users found frustrating about EMAN2.0, and we strongly encourage people to upgrade.
EMAN is a suite of scientific image processing tools aimed primarily at the transmission electron microscopy community, though it is beginning to be used in other fields as well. For example it can do an admirable job aligning images for amateur astronomers. EMAN has a particular focus on performing a task known as single particle reconstruction. In this method, images of nanoscale molecules and molecular assemblies embedded in vitreous (glassy) ice are collected on a transmission electron microscope, then processed using EMAN to produce a complete 3-D recosntruction at resolutions now approaching atomic resolution. For low resolution structures (~2 nm), this may require ~8 hours of computer processing and a few thousand particles. For structures aimed at ~0.5 nm or better resolution, hundreds of thousands of particles and hundreds of thousands of CPU-hours (on large computational clusters) may be required. Indeed, EMAN is often used in supercomputing facilities as a test application for large-scale computing.