If you are whipping up some JavaScript and want a great place to build/run and perhaps share with other, then you need to check out JSFiddle.  This tool is fantastic, and a great way to collaboratively work on JavaScript.  For debugging and/or writing out to a console, consider using Firefox’s [awesome] firebug addin (IE Developer tools works pretty well too).

For times when you need to use regular expressions – and you’re either not strong at them, or have been away from them for more than 2 minutes (the time it takes to forget most of the syntax), then check out RegExPal.  This little utility is handy, and give great real-time visual indications of matches.