Jitbit's list of the most frequently used online development tools:

Javascript compressor - a great tool to compress and obfuscate your Javascript code. Produces 2-10 times smaller scripts, speeding up your web-applications significantly. Highly recommended.

Google translate - yep. Both ASP.NET forum and helpdesk software are multilingual, so adding a simple label or a button to these products becomes a headache... Or - would become a headache, if there was no Google. Great tool. We even designed an internal utility that translates an ASP.NET resource file to a number of specified languages via Google-translate. We even consider releasing it to the public under some open-source license.

VirusTotal - a nice place to check your files for antivirus false-alarms. You never know when some paranoid antivirus will mark your program as a threat... For example, old versions of our Macro Recorder were accessing the registry using "ntdll.dll" and a number of anti-viruses have marked that as a suspicious behaviour. Always check your releases there.

IconFinder - not exactly a tool, but a great resource for UI-development. Hundreds of icons for your applications, both free and commercial.

AjaxLoad - a great online tool to create an animated "loading" icon for your Ajax-applications.