iBorrowDesk - iBorrow Desk is a tool for monitoring borrow rates and availability using Interactive Broker's freely available data.
Short Squeeze - ShortSqueeze provides short interest data and related services.
YAHOO Finance - Yahoo Finance is a classic. It provides free real-time and historical stock quotes and charting, news etc.
ROIC - ROIC provides 30 years of financial data and news feed on 7,000 companies. ROIC is free.
Most tools under “Full service research tools” also have market and financial data.