Most PC users probably have only the primary PCIe slot populated with their graphics card, with the rest of the x16, x8, x4, and x1 slots remaining vacant. Modern motherboards already provide onboard Wi-Fi, plenty of USB ports, and more than enough M.2 slots, so most people don't feel the need to add PCIe expansion cards. That said, you could still run out of ports and expansion slots, or you may want to switch to more advanced NICs without changing your motherboard. Your free PCIe slots can come in clutch in such situations, offering relatively affordable and seamless upgrades for your system without the need for major hardware replacements. You may not have considered expansion cards for your PC yet, but this list may inspire you to do so.

HBA cards

For those who take their home servers seriously

Technically, a Host Bus Adapter (HBA) card is a dedicated interface between a host system and storage or networking devices. It has its own processor and firmware so that the host system's CPU is freed up from managing data transfer or networking tasks. For a home server, a SATA or SAS HBA can connect tons of hard drives or SSDs to the system while sidestepping the complexity and hassle that come with cheap add-in cards and port multiplexers. These HBAs expose 8, 16, or more drives directly to the OS without abstraction layers or RAID processing in between.

On the other hand, more expensive networking HBAs use Fiber Channel (FC) to offer block-level access to storage, a common practice in enterprise systems. FC HBAs are overkill for most home servers, but if your system has expanded organically over time, you may still find it useful. FC cards generally use a x8 PCIe slot that has PCIe 3.0 or PCIe 4.0 support for ideal bandwidth.

USB expansion cards

No more rotating devices

Running out of USB ports is common on budget motherboards, where even the basics can saturate your I/O. A keyboard, mouse, headset, speakers, external drive, and some dongles are enough to populate all your rear USB ports. If you need to connect more devices or have already transitioned from a regular PC to a home lab, you'll need some reinforcements, unless you want to use your PC's front I/O, which isn't ideal. This is where PCIe USB cards come in, typically providing 4–5 USB ports on a single PCB. You just need a x1 PCIe slot to connect such a card. Instead of a USB hub, a PCIe card fully utilizes the available USB bandwidth. Also, it's a less cluttered solution. You no longer need to remove some devices to plug in your VR headset or an additional drive.

NVMe adapters

Add more NVMe SSDs to your system

If you've already populated all the M.2 slots on your motherboard, and don't want to replace existing drives with new ones, a PCIe to M.2 adapter may be your only choice. These adapters allow you to add one or more M.2 SSDs on an expansion card installed in a free PCIe slot on your motherboard. Since the NVMe interface uses the PCIe bus anyway, the performance you get from a decent PCIe to M.2 adapter is indistinguishable from a direct M.2 connection. Some adapters even improve the read/write speeds over the default M.2 connection, especially in systems where the stock heatsinks aren't able to keep the drives cool for maximum performance.

PCIe add-in cards also offer PCIe bifurcation to distribute lanes between multiple SSDs installed on the same card. As long as your system has enough lanes to offer, your PCIe to M.2 adapter won't suffer from any bandwidth limitations or interfere with the GPU performance in the primary PCIe slot.

10GbE NICs

Unlimited power

While the average user doesn't need multi-gig Ethernet for most uses, power users or home labbers are already using 2.5GbE and 5GbE LAN to support massive file transfers, NAS workloads, automated backups, and more. If you need to enhance your old motherboard's networking capabilities, you can pick a 10GbE NIC for the job. Sure, it won't be as cheap as slightly slower adapters, but upgrading to 10GbE can be amazing. Your self-hosted services, virtual machines, and smart devices can gradually strain your home network. If you're not averse to some future-proofing, installing a 10GbE NIC isn't that overkill for such a setup.

You will need to ensure your other networking equipment keeps up with your 10GbE NIC, though. A faster switch could ramp up your investment significantly. However, if you're already running a home server that demands 10GbE networking, you may already have a faster switch on your mind. An advanced NIC is one of the most powerful uses of a free PCIe x4 slot.

PCIe offers tons of expansion possibilities

Your motherboard's PCIe slots aren't just for installing graphics cards. Most users will have at least 3–4 empty PCIe slots that can be used to expand their PC's functionality beyond what they initially imagined. Multi-gig network cards, NVMe adapters, USB cards, and HBA cards are some of the most useful additions you can make to your system, especially if you're a power user who keeps running out of ports and slots on your motherboard.