Webresizer

A great online tool for optimising your photos (will even make rounded edges!)

Picmonkey

A stunning peice of online photo manipulation.  Upload your image, apply the (almost Photshop-like) effect you want and download to your computer.  Some of the effects aren't free.

Fotosizer

Fotosizer is a free batch photo/image resizer tool. It lets you resize hundreds of photos in a matter of minutes in a quick and easy way.  It is afree programme which you download.

LunaPic

An online image editor that claims to create transparencies - very useful for websites.  It's quite complex to use at first.

Fotoflexer

You can make a wealth of changes to your photos here.  It's an online photo editor... But is picmonkey better?

a list of 35 or so online photo editing tools

It doesn't have all of them, but it will keep you busy!

Pho.to

Pretty slick online photo editing tool.

Photoshop Express

This is a cut down version of photoshop express online.  You don't have to register to use it... it's free.