Javascript-only tools that I frequently use:

string functions

• hex / oct / dec converter (really though, just use Google Calculator)
• base64
• base64
• base64
• base36

programming

• URL extractor
• URL encoder / decoder
• un-minify Javascript
• unix time converter

math

• decimal to fraction converter

finance

• loan calculator

less-frequently used tools

• RPG/dice probability calculator
• chmod calculator

"Online" means tools that run as 100%-javascript as possible, and avoid sending proprietary information to the server.
This makes them useful when working in HIPAA / PCI DSS environments.