Mac driver for USB Ethernet adapter
I recently got one of those new macbooks without a built-in ethernet port. At first I thought I could simply plug in an old USB to ethernet adapter (Linksys USB100TX) from long time ago, and I would be able to connect with a wired connection - after all, it's an old USB device, it should just work right? Nope, it turns out there's no official support for third-party adapter, and it requires a separate software driver 
.
Luckily...
After some googling, I came across http://www.sustworks.com/site/news_usb_ethernet.html. Apparently most older USB Ethernet adapters use the same chipset and can use the same "Pegasus driver". Unfortunately, the driver on the website only supports up to Tiger, and does not work on my Mountain Lion machine out of the box.
The author has kindly included a copy of the source in the download, and it turns out the driver almost worked out of the box: it needs to be re-compiled against the Mountain Lion SDK and in 64-bit (there's a small 1-line change that needed to be made as well). After installing it with kext wizard, I'm connected!
Yay, saved myself some money and not having to buy a new adapter.
Download
Here's the version that works in Mountain Lion. All credits go to the original author at sustworks, I simply did some minor changes but thought I would share it since I'm sure other people would run into the same problem:
USB-to-ethernet adapter driver ("Pegasus Driver") for Mountain Lion
Simply extract the .zip, and install the file using kext wizard.