Original Airport Card Alternatives
A few years back Apple stopped selling the original Airport card because (so I was told) as the manufacturer had stopped making the card.
If you have a Mac which requires an original Airport card there are still a few suppliers willing to sell them, or you can get them off eBay.
Note that you can not replace an original Airport card with a new Airport Extreme card as they each have a different format and the slots required are very different.
There are also alternatives which can either be used instead of an Airport card or to provide the Mac with 802.11g speeds over the 802.11b speeds of the original card. The alternatives depend on which Mac you have.
PCCard (or PCMCIA Card)
Some 802.11b PC Cards will work in a PowerBook, but unless you know for sure they will work natively under OS X, ensure that the manufacturer providers Mac drivers.
The only option if you have an old PowerBook to upgrade to 802.11g is to purchase an PCMCIA 802.11g card. Or you could use a 802.11g wireless bridge (but this is not very portable, but can be a solution for an old iMac). You want a third party WLAN Cardbus cards based on the same Broadcomm chipset that Apple used in the Airport Extreme which will work in a Mac running OS X 10.2.6 or later and Airport 3.1 or later.
Companies using the Broadcomm chipset include: Linksys, Buffalo Technology, Belkin.
However another option is to use an Atheros card and the Orangeware software. The website lists the compatible cards.
If you have an older PowerBook then this guide: Wirelessly Networking a PowerBook 1400 or Other Old Apple Laptop: Step By Step is certainly useful.
Wireless Ethernet Bridge
These are driverless so will work with any Mac with an ethernet port. Basically they allow any network device with an ethernet port to join a wireless network.
I have used them with non-wireless computers, the Elgato EyeHome and a Keyspan USB Server.
Examples of Ethernet wireless adapters are the Linksys WET11 which is 802.11b and the Linksys WET54G
There are a couple of products which allow you to connect up to five ethernet devices via a single wireless bridge, these include the Buffalo Technology WLI-TX4-G54HP bridge
PCI Card
If you have a PowerMac with PCI slots then you can use a PCI card. You want a third party WLAN Cardbus cards based on the same Broadcomm chipset that Apple used in the Airport Extreme which will work in a Mac running OS X 10.2.6 or later and Airport 3.1 or later.
Companies using the Broadcomm chipset include: Linksys, Buffalo Technology, Belkin.
However another option is to use an Atheros PCI card and the Orangeware software. The website lists the compatible cards.
Airport Express in WDS Mode
When an Airport Express Base Station is used as a WDS node the ethernet port is active and can be used as a wireless ethernet bridge (just like an etherent wireless bridge).
This is certainly an option if you have a spare Airport Express.
Due to the bandwidth hit that you get with WDS, I wouldn't recommend them over say an ethernet wireless bridge.
Wifi USB Adapters
I would avoid USB wireless adapters they are just trouble and as they rely on drivers these can be "broken" with an OS update. Obviously for an iBook they are the most portable option.