I wish to find some software to prepare schematic illustrations for
publication in journal papers.

I would like to know which packages are available, any independant
reviews or comparisons of alternative packages, prices and where I can
purchase them online.

Further information: The types of illustrations I wish to create
include things such as schematic layouts of water treatment plants, or
environmental systems (eg. the nitrogen cycle, etc..). I wish to
create good quality diagrams suitable for publication in books or
scientifc journals.

Thanks!

For what little it is worth:
The best package for this, EVER, in my opinion was called
"Intellidraw".  It was a Mac/PC program from Aldus, but was dropped
when Aldus was acquired by Adobe.  Nothing I have ever seen since
(that is in over 10 years) can match it for versatility.  I first
began using it on a Mac Quadra 700 under OS 7, and I still use it
after 3 hardware changes and 3 operating system revisions.

If you EVER see a copy of it anywhere, get it.  The learning curve is
a little steep, but the power is phenomenal.  It was a dark day (and a
ridiculous decision) when Adobe dropped it.  Their mainstream product
in the same line "Illustrator", is functionally deficient bloatware in
comparison.

Any cheap CAD program (Computer-Aided Drafting) will do it for $50 to
$100. You can find them at any computer store or Office Depot. Also
check Download.com for free versions.

Well, for my papers I mostly use xfig. You can also try a new software package
called Dia. Xfig is quite practical and you can install it under windows,
see http://www.cs.usask.ca/grads/wew036/latex/xfig.html. I also found 
WinFIG at http://user.cs.tu-berlin.de/~huluvu/WinFIG.htm. Anyway, I use the linux
version of xfig so I can't give any specific recommendations for windows, but 
it is free (so you might want to give it a try) and it is reasonably easy to use.
Dia is a newer xfig alternative and is also free and available for
windows via a conventient installer. See
http://dia-installer.sourceforge.net/. Most users would probably pick
Dia over xfig but I still prefer the latter.

You could try more expensive "vector" drawing programs for better
results but I don't think it is worth the expense.

I recommend Microsoft Visio for what you are trying to do. Microsoft
Visio 2002 (the version I use) has templates and libraries to make
Process Engineering drawings (for Piping & Instrumentation diagram/
Process flow diagrams). For making environmental system drawings, you
can use their map category which has a directional 3d template with
some nice graphics. Visio is very simple to get used to, and has lots
of other features like importing and exporting to CAD, and other
Microsoft document softwares (good for publishing). It also has
templates for block diagrams, building plans, electrical engineering
schematics (my favorite), mechanical engineering, and many more.
You can download a 30 day trial from the following link:
http://www.microsoft.com/office/visio/prodinfo/trial.mspx