We use the following free ones:
squirrelmail (see above for url)
horde (horde.org)
neomail (neomail.org)
Another product to look at:
hivemail (hivemail.com) - not free but quite an intresting project going on there.
We also use a webmail service found at:
http://webmail.agcc.net
At the moment, I cant renember where I got it from, but if its of any use to you, let me know and ill send you the script.