A network tap is a hardware device with the capability to provide passive access to packets on the wire. Those are mostly used for IDS, RMON probes, Traffic Collector, Packet Analyzer, but sometimes required to aggregate flows from more than one sources into a single data stream. Taps are also useful, if you have Dual-Homed Server connected on two different switches. With that, you can easily mirror both NICs and aggregate them together via a TAP and then connected to a Network Analyzer for further investigations. Also considering TAPs, when having asymmetrical traffic or get traffic from two routers
There are many more options to use Taps, even just when simply no SPAN port is available, or you want to add an additional IDS to it.
Some vendors, I came across or were mentioned to me, are linked below