First, Gigabit Ethernet doesn't allows hubs. When the IEEE first defined GigE, they briefly had a spec for how a GigE hub should work, but no one ever shipped one, and the IEEE quickly deprecated the spec and recommended that GigE always be switched. (Trivia note: This means that GigE is technically not CSMA/CD.)
You can still buy 100BASE-TX hubs if you know where to look, especially so-called "dual-speed hubs". A 10/100 dual-speed hub is effectively a combination of a 10BASE-T hub and a 100BASE-TX hub in the same box, with a 2-port bridge (switch) chip in between the two hubs. If a 100BASE-TX device is connected to a port, that port gets connected to the 100BASE-TX hub. If a 10BASE-T device is connected to a port, that port gets connected to the 10BASE-T hub. So all 100BASE-TX devices can snoop on each others' unicast traffic, and all 10BASE-T devices can snoop on each others' unicast traffic. But the 100BASE-TX devices can't snoop on the 10BASE-T devices' unicast traffic (or vice-versa), because there's a bridge in between.
Since you probably don't have an 10BASE-T-only equipment around anymore, a dual-speed hub is going to be exactly the same as a pure 100BASE-TX hub for your purposes.
And, of course, as others have pointed out, manageable switches often allow you to set up "port mirroring", also known as "port spanning" or "sniffer port" in some products, which allows you to make sure that one port sees all the traffic to/from another port on the switch, for the sake of sniffers and other traffic monitoring tools.