Paste your source code and this site will run it and provide output.  38 languages are supported.
ideone.com

Type in a regular expression and a block of text to see what it matches.  Also provides a large database of community submitted regexes (under the community tab).
www.gskinner.com/RegExr

Minify your website's JavaScript files to decrease load times.
javascriptcompressor.com

JSON is a simple serialization format for sending complex data between programs or across a network (often for websites).  This tool allows you to paste unformatted json text and adds line returns, indentation, and coloring for easy-readying.  It also validates.
jsonformatter.curiousconcept.com

ICO files are used for Windows icons and also the icons seen next to sites in the address bar.
convertico.com

An online guid generator for when you can't remember where Microsoft puts that command line utility.
www.guidgenerator.com/online-guid-generator.aspx

shell-tools.net/index.php?op=xml_format
Paste unformatted xml and get back a neatly-aligned document.

An onlinve validator to make sure xml is well formed, and also provides the option for validating against a DTD.
www.xmlvalidation.com

Encode and decode between these formats.
ostermiller.org/calc/encode.html

A unix timestamp is the number of seconds since Jan 1, 1970 and is a common way to stare dates.  unixtimestamp.com provides the current timestamp and a calculator for other dates.

Validate the html code of your website to check for errors.
validator.w3.org

Includes ip address lookup, ping, whois, reverse dns, traceroute, and more.
iptools.com

The BMP contains characters for almost all modern languages, and a large number of special characters.
en.wikipedia.org/wiki/Mapping_of_Unicode_character_planes#Basic_Multilingual_Plane