There are a few websites that I find useful.
Database designer, using Google Apps. Allows you to build a database diagram and output it in code that you can use to directly create your MySQL database. There are more complete or easier apps to use, but it's pretty cool when you can use something like this, and use the resultant database in something like Gii and have the basis of a PHP application with CRUD, search and view forms with only a database diagram and a few clicks!
This next website is really nice. It allows you to take any image, preferably a GIF or PNG with transparency (although it can use a fixed colour background), and mark-up the individual icons or graphics (sprites) in the image. Great if you're working on an application with a lot of buttons and/or interactivity.
This last one is probably my favourite of the three. It shouldn't be underestimated what can be achieved with some carefully crafted regular expressions. This site has great tools to build up regular expressions, and you can copy in text and test it out in place. Useful in itself, but there's also a way to share and rate other regular expressions, and it can be a great resource, or at the very least a fantastic start.
Other than that there are a few scripts that I like to use, but those require running on a localhost as they would be impractical or insecure to be used on a live website. My favourite script is one that allows me to run arbitrary snippets of PHP code, and I may include code to it at some stage in the future. I'll also post about other applications and tools that I use offline for developing websites.