ARM-JTAG-COOCOX is an ARM debugger based on CoLinkEX design. It is meant to work out-of-the-box with the free CooCox IDE. It can also be used with Keil MDK and IAR EW IDEs (the driver for IAR is still not perfect) via external plug-ins. Its affordable price and the ability to debug via both SWD (Serial Wire Debug) and JTAG interfaces are the key points.
Open hardware - you have access to board design and board schematic
Supports Cortex-M0, Cortex-M3 and Cortex-M4 devices