I used to play Warhammer and Mordheim games using the system outlined here:
http://z14.invisionfree.com/The_Lost_Glade/index.php?s=8e114c6970e572cb4c9958e189848261&showforum=30Unfortunately that site is now old enough that all of the actual battle threads etc. have been lost, but the basic idea is simple- use a program with graphics layers to move units about the map, rolling the dice either yourself in real life or using an 'online dice roller' if you don't want to rely on trust (online rollers often produce a trackable document you can use to prove the die rolls).
It's obviously a pretty crude system, but that also made it easy for anyone to play- and completely free.
The main problem was speed of play (improved if you play by MSN, or with very regular forum goers, or by organising a time when both users are active on the forum).
Edit:
This is an old pic of an in-progress game

A grid overlay is used to measure movement, but it's easier to see the action by posting the image without it. Both players work off the one map and grid combination to make sure there's no mis-alignment or stretching issues (ie. One player posts each layer making the map and grid, the other player copies them)