An integrated logic analyzer is a useful tool for debugging FPGA designs. Integrated logic analyzers allow the the capture and examination of signals in a design while using the actual hardware device. It is very useful for finding and solving unforeseen issues with large and complex designs.
Both Xilinx and Altera have designed ILAs for their respective chips. Xilinx's ILA is called Chipscope. In addition to an ILA it also has a VIO (virtual IO core) for changing signals in real time, embedded processor bus analyzers, and high speed serial bit rate tests.
Altera's ILA is called Signal Tap.
Integrated logic analyzers use FPGA resources when instantiated. The logic analyzer is created using FPGA fabric and the samples are often stored using the hard core RAM blocks in the FPGA. Using an ILA can prevent these resources from being used by the actual design and inclusion of a ILA can make the the design harder to meet timing during a place and route.