Atheros and Madwifi is 'ok' nowadays. It used to be that they were the only game in town, but since then things have changed. They are still very nice, so don't get me wrong. Hopefully the completely open source versions will solve what is left of the usability and supportability issues.
Depending on what you want there are several choices. If your purchasing a laptop then definately you'd want Intel. In new laptops they've moved from Mini-PCI to Mini PCI Express and Intel has two cards for that. The first one is the 3945ABG card. and the second one is the 4965AGN card. The 3945ABG has 2 versions of drivers for it, each requiring a different firmware image (which needs to be installed seperate from the kernel drivers). The more common version is the ipw3945 driver. This driver is what will be in most distributions. It requires the use of the proprietary 'regulatory' ipw3945d daemon to control the card, but the drivers themselves are completely open source.
http://ipw3945.sourceforge.net/
For the 4965AGN card they use the newer iwlwifi drivers. Iwlwifi drivers have support for both the 3945ABG and the 4965AGN wifi card. These do not require the regulatory daemon since the FCC stuff has been moved to the loadable firmware of the device. I prefer the iwlwifi drivers since I don't have to deal with the daemon and they've been more stable in my experiance.
http://www.intellinuxwireless.org/?p=iwlwifi
If you purchase a laptop then definately Intel is desirable. Intel has their 'Centrino' brand-name, which means Intel CPU + Intel Chipset + Intel Wireless. The video can be ATI, Nvidia, or Intel. If your purchasing a laptop and the wireless is labelled 'Dell' or is called 'Broadcom' then these are not desirable. You want to make sure to get Intel wireless and often companies will use Intel in one version of a laptop and Broadcom in another. It's something to be carefull of. Different trim levels may have different wifi.
For add-on cards I like the Ralink hardware. They preform well, are inexpensive, and have good open source driver support. Support for most cards should be in the 2.6.24 kernel. Older devices do not require a firmware image to be installed, but newer stuff does. There are two versions of drivers avialable for Ralink also.. There is the older 'Legacy' drivers based on drivers that are (looks like) XP drivers ported Linux and then open sourced by Ralink. These are supported in 'bugfix' mode. The newer drivers are like the iwlwifi stuff and are based on the new m80211 protocol stack that was recently added to the kernel. Mac80211 protocol stack enables many extra features and simplifies the drivers as much as possible and this is desirable. They have a bit better performance, use less ram, etc etc. But either one can work fine. For the 'Legacy' drivers they have a special GUI made called Rutilt that you need to control the wifi. The older drivers are not compatable with Network-manager so that needs to have 'networking' turned off and you need to bring the 'device up' before you can use Rutilt. But the newer drivers don't have this problem.
The driver project is found at:
http://rt2x00.serialmonkey.com/
For finding out which chipset is being used with which I like this page:
Ralink:
http://ralink.rapla.net/
Atheros:
http://atheros.rapla.net/
Intel:
http://intel.rapla.net/
And there are others. If you look at the bottom of one of those rapla pages there are links to others.
BE VERY CAREFULL. Something like a Linksys 12345 can use a entirely different hardware from Linksys 12345v1.1 and require entirely different drivers. One may work perfectly in Linux, but the other be very difficult to get working. And often the box art and numbers on websites won't reflect any differences. So sometimes it's a PITA to find the desirable cards.
The one thing for Atheros is that there is no USB driver support for Linux. For both Ralink and Atheros there is no support for mini-pci express drivers yet, or at least they are in the very experimental drivers.
I own a new just-got-support Asus EEE PC with madwifi drivers. I had to pull patches off the online bug logging system to make those work.
I have a Rt73-based USB device I use the newer Rt2x00 drivers on.
I have a Rt2500 PCI device that I use the legacy Ralink drivers on.
I had a Rt2500usb device that I used both the legacy and the newer drivers on.
I had a Apple Ibook with a broadcom (Airport Extreme) wifi that I used in-kernel drivers for. (ran linux on it till it crapped out due to hardware failure)
I had both a cardbus-based and PCI Prism54-based device that was very good, but a bit expensive, until Conextent changed the hardware and lied about it. Mine stills works, obviosly, but they are nearly impossible to find and if you do it's impossible to tell the 'Good' hardware mac version vs the 'Bad' software mac version. Conextent told manufacturers that it was a drop-in replacement with no hardware changes, but they lied, so it's very hard to find the desirable ones.
So thats what I've used. Besides that there are good drivers for Realtek and Zydas devices that should work (and probably others), but I have no personal experiance with.