What's a good tool to create Sankey diagrams? How can I create Sankey diagrams? 
 A: Have you seen this list?  And there is also a function in R available.  I personally would start with the path geom and size aesthetic in ggplot2 and see where that got me.
I haven't tested any of these.  If you find a preferred option perhaps you could let us all know as they are rather cool graphics.
A: If you are looking for client side (JavaScript library) you can try:
http://tamc.github.com/Sankey/
You can also see a related question on StackOverFlow: https://stackoverflow.com/q/4545254/179529
A: Check out my HTML5 D3 Sankey Diagram Generator - complete with self-loops and all :) http://sankey.csaladen.es
A: Are you looking for library or web app? If the second, you may try this Sankey Builder. It has visual interface and drag&drop UI. It is just Beta, at this point it only saves data to it's URL and is not optimised for mobile. 
http://wikibudgets.org/sankey/
disclaimer: I work for wikiBudgets 
A: I just uploaded a brand new online Sankey Builder. You can upload data, configure via a wide range of tools and save. It allows you to drag and drop fields from your data to customize the Sankey Flow and dynamically add filters on any field in the diagram to squeeze the data. It has automatic highlights of bands across the diagram to highlight data relationships, and you can even mix and match colors. It features automatic paging for large data sets! A sort feature operates by value or field in ascending order.
All settings can be saved for a future visit.  The tool includes a share feature which allows you to enable or disable any of the settings.  The unique URL created by the tool allows you to distribute the Sankey Diagram to visitors for a read-only interactive version of Sankey Builder!  Check out the demo (see below).  You can even build a free Sankey diagram which is hosted for free. Simply visit http://SankeyBuilder.com and signup (all you need is an email address).  That will get you access to SankeyBuilder for free. Demo: http://sankeybuilder.com/sankeybuilder.aspx?url=bbe34d97f8 and Tutorials are at http://sankeybuilder.com.
Hope this helps.
A: Our Sankey Diagram app is for Apple iOS.  It uses the touch interface to provide intuitive creation of flow diagrams. Just search for "Sankey Diagram" in the Apple iTunes app store.  Our web site is squishLogic Sankey Diagram 
A: Draw Sankey diagrams directly in your browser with "Sankey Flow Show - Attractive flow diagrams made in minutes!"
http://www.sankeyflowshow.com

A: Like everything Latex is the way !
It you do not know latex at all you might not want this solution, but if you know even just a little, you just to have to change parameters accordingly to what you want, and it will be more precise and flexible than many other possibilities.
Note that you will have to enter the values manually.
You can use the Tikz library to produce that
Here is the code
A: We build on Denes's online (web app) Sankey Diagram Generator with some more functions and abilties to copy data in from CSV, Excel Pivot Tables, and JSON.
Sankey Diagram Generator
