The BU-303 is shipped by two companies so far as I know. They are Deluo and NaviLock. They cost about 90 euro's in the Netherlands and are available from any retailshop that get's their stuff from JavoCom The driver for the BU-303 for Mac OS X can be found here (29-03-03: Link Fixed!). Actually it is not the driver for the BU-303, but for the Prolific 2303 chipset that is used on the BU-303. This chipset is actually an USB-to-Serial convertor that is needed to support the serial SiRF chipset (the actual GPS chipset) on the BU-303.
It works fine on Mac OS X with this driver. I have no windows machines that have need for the GPS and I have never used it on Windows. Usage under Linux is possible with in-kernel drivers, however the Linux 2.4 driver is very unstable and can lead to crashes after using the GPS for a while. According to Greg Koah (of Linux USB fame) this has been fixed in the Linux 2.6 kernel. However I have NOT tested this.
The BU-303 works fine using KisMac and works correctly using Route66, however the update interval using Route66 is rather slow. This is probably Route66's fault AND NOT the fault of the BU-303 GPS.
UPDATE: (03-08-2004): I see that a lot of people make use of my BU-303 article in my weblog. At the moment there is only a download for the Mac OS X driver, but with this overwhelming hits on BU-303 (according to referrers). I will make more drivers available here in the future.
UPDATE: (20-09-2004): Thanks to Jaap there is a new version of the BU-303 available. It is downloadable here, this is version 1.0.8, the old version is 1.0.6. Also Jaap found another driver for the BU-303 which can be downloaded here which he found at MacGPSPro . I Haven't tested the drivers myself as I don't use my GPS that often and the current driver works just fine for me. Thanks Jaap!
UPDATE: (20-11-2004): It seems there are some BU-303 units which have faulty soldering bonds. I found this link describing how to fix this issue if it happened to your unit. The page also has some more technical details about this unit.
Also some links to articles linking to my entry here: