Outside the Apple universe, there are numerous things to consider. If you want a wireless router purely to connect a laptop and a couple of other devices to the internet, forget about data throughput. As we explain in the WAN section, your internet connection is the bottleneck there, and upgrading your wireless network won't help.
What might assist matters is a dual-band router; if you have devices that support 802.11n, look for a router that supports the n standard. Separating 2.4GHz and 5GHz devices reduces interference and improves the performance of your network. But avoid routers that support only 5GHz - unless you know you'll never have to connect a 2.4GHz device.
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For flexibility, a router with four Ethernet (preferably gigabit rather than 100 Base-T) ports will allow you to connect devices directly with Ethernet cables. This might be more important than you think. If, for example, you have a Virgin Media TiVo box and want to use the Virgin Media Anywhere iOS app to control it, the box must be connected to your network by cable. Similarly, if you have a set-top box, smart TV, or games console that doesn't have built-in wireless capability, or supports only b or g, you may be better off hooking it up to your network directly (assuming it has an Ethernet port) rather than buying a separate Wi-Fi adaptor.
Talking of ports, some routers, like AirPort Extreme, have a USB socket. This is good. You can either hook up a printer and make it accessible to Macs and PCs on the network, or attach a hard drive and share its files over the network. We'd recommend a dedicated NAS box, not least for the extra features it offers, if you plan to share files on a network, but a USB hard drive attached to a router is great for occasional use.
The ability to create a guest network is a pretty clever feature, too. Did we mention that AirPort Extreme can do that? A guest network allows you to give visitors to your home or office wireless access to the internet, without letting them roam freely over the rest of your network.
Support for WPA2 security should be a given, but double-check to make sure. And a decent router should also have a WPS (Wi-Fi protected set-up) button to allow you to connect compatible devices without too much fiddling.
Internal or external antennae? Excellent question, glad you asked: most routers now come with the antennae hidden away inside the case. That, of course, makes them more aesthetically appealing - a not-insignificant consideration, given what we've already said about placement.
Routers with external antennae, however, do have a couple of advantages. The first is that you can adjust an antenna to improve the signal, though in reality this is likely to make little difference. The second is that you can replace the antennae with third-party versions; these might allow you to place them away from the router, say, higher up, and improve the signal.
That's an additional expense, however, and again, not likely to improve matters a great deal, so don't fixate on getting a router with its antennae on the outside.
Don't ignore travel routers. If your purpose in buying a router is to create a bridged wireless network or to act as an access point to an internet connection rather than to connect multiple devices using Ethernet, it might be worth considering a travel router. For obvious reasons, these boxes are smaller than regular routers, and typically have fewer Ethernet ports and smaller antennae. But they're very capable and have the advantage that they can be taken with you, so if you find yourself in a hotel room with wired internet access you can create your own in-room wireless network.