Here is a snort list of useful, off the shelf, network devices that sell for around $50. Not necessarily stuff that you would want use long-term in production, but more for the one-off projects or for troubleshooting. 

First off is a 5-Port Gigabit switch from Netgear (the GS105Ev2).
It is interesting because it supports port mirroring and only costs about $35. That’s a really good price. It obviously isn’t going to replace a real network tap and you will lose traffic if the upstream plus downstream you are copying are together more than 1Gbit of traffic (but that’s a problem you always have when mirroring ports 1:1). It is dirt cheap, fanless and small enough to throw in a bag, can’t go wrong with this one. Need a temporary tap to monitor traffic? Just pop it inline and you are good to go.

If you are more of a DIY kind of person, I’d suggest the ESPRESSObin.
It sells for about $50 and is a single board computer with Gigabit network interfaces and a SATA interface. It is based on ARM architecture and runs Linux.
Since it has multiple Gbit interfaces and you can hook a HDD up to it you have a wide array of possibilities you could use it for. A lot of network traffic monitoring situations come to mind, where you would want to keep copies of the traffic in question for later analysis.

Another little useful device is the EdgeRouter X from Ubiquiti.
As the name suggests it’s a router and sells for about $55 The nice thing about this device is that it runs Linux (or more specifically a fork of Vyatta), so you can dig into it to get it to do things not available on the GUI.
5 Gbit ports, RIP and OSPF routing protocols, usual SOHO features like Firewall, VPN, QoS. Behaves a lot like a switch with some routing functionality. Unfortunately I couldn’t get port mirroring to work reliably but aside from that it’s still an impressive little router.