Umbraco tip of the week - Debugging from within the website
Not only can you debug your umbraco site by adding "?umbDebugShowTrace=true" to the current URL, but you can also show a list of element in the page using "?umbDebug=true".
Using the URL rewriting feature, you can also disable the debug querystrings so that you don't disclose too much information on your live server, see the Umbraco book on how to do this.
I was developing a custom module earlier this week and noticed that I could just add a trace.write statement to see why it did not work on the live server. Turns out that the problem was somewhere else completely, which I quickly found out once I started adding some trace statements. Nice!