Here is a list of all the web based software that I've used or evaluated (or is on my list to evaluate).
RefactorMe - This page is ugly and needs to be broken out.
See also: BlogSoftware, WikiSoftware, BackupSoftware
Image Gallery Software
BB Gallery - http://www.bb-zone.com/zope/bbzone/projects/bbgallery
Curator - http://curator.sourceforge.net/
Email Software
My personal pet peeve with web based email clients was that so many of the required a SQL backend. I can only say "what the hell for?" This list deliberately excludes any web based email client which requires a SQL backend (of which there are several nice ones). I wanted an program which used IMAP (using POP3 with web clients has issues) and supported the ability to maintain an addressbook, deal with attachments and was reliable and fairly pretty.
Squirrel Mail - http://www.squirrelmail.org/ Squirrel Mail is great. It's my number one choice.