In yesterday's post about things you can't yet do in a web browser, Tyler mentioned that code development is one of the major existing deficiencies in web-based software. While it is by no means a full solution (or even a partial one, really), Codepad.org can be an occasionally useful tool for development in a number of popular languages.
Similar to previously mentioned Regexr, Codepad lets you upload or write a snippet (or larger piece) of code. This code is then run on the server, generating a page with a view including the source code, as well as the output of running the code. The resulting page is posted to a webpage that you can then share with collaborators (great for getting some debugging help). Here's an example page for a php "hello world" script.
Codepad defaults to making all of your code public, but it also provides a "private" option that is protected by a cryptic url that you can share with others (it's worth noting that this really provides on a minimum level of privacy). For bigger projects, or for a compiled listing of related pieces of code, you can also set up a project page (again, as either private or public).
There's certainly nothing particularly flashy about the codepad, and it isn't something you're likely to need every day (even if you are a developer). For those times when you need to get a little help with some code, want to share some code with another collaborator, or just do a little debugging on a machine without access to a particular programming language, codepad is great.