Field Programmable Gate Array (FPGA)
FPGAs are useful to systems designers that make their own chips. The engineers will try their ideas in FPGA form first, to accommodate any changes or debugging, later transferring the project to an ASIC.
Sometimes, a vendor will release a system or linecard that's still based on FPGAs, promising that a slimmer, future version will be based on ASICs.
FPGAs used to be compared to cassette tapes that start blank and can have music recorded and re-recorded on them. These days, it's more appropriate (but less satisfying) to compare them to empty iPods.