2 letters.  DD     

A book written by a buddy of mine may be something you'd enjoy:

http://www.amazon.com/UNIX-Linux-Forensic-Analysis-Toolkit/dp/1597492698/ref=sr_1_1?ie=UTF8&s=books&qid=1241936083&sr=1-1

It mentions ddfl-dd (dd that also cuts md5 on the fly),  EnCase's LinEn, Access Data's FTK Imager, and ProDiscover as options for imaging.   Encase forensic edition apparently remains the pro's choice but does cost a lot more than "free."   

You may be interested in the Helix distro of Linux, but I think they may have gone non-free here very recently:
http://distrowatch.com/?newsid=05102

Whatever you use, what's most important is to make certain that your image includes all slack space, and can be verified (via md5 or shasums) to be identical to the original disk, chain of custody maintained, preferably image taken with write wires cut, and all that good forensics guy doo dah stuff!

There are very few people actually doing imaging with EnCase products, including Linen.   They are painfully slow.   The Raptor disc is able to create E01 images, much much faster than EnCase or Linen can.   We get about 2 GB/min on decent hardware.   EnCase is nowhere close to that.

Of course, nothing beats EnCase for doing actual analysis.