I realize that there's been a lull in this thread, but I wanted to mention MacGPS Pro as an option. I'm a consulting archaeologist, and I've been using the program for several years for surveying and mapping. It's compatible with most Garmin gps units (I use a somewhat antiquated Garmin 12), and data transfer between the gps and program is seamless and simple. I also use it for real time tracking on my laptop while surveying - again, simple to do - just hook up for GPS and hit the Real Time Display menu item. MacGPS Pro also has the ability to save files in several formats, including Google Earth-friendly KML. In addition to importing georeferenced maps, you have the option of manually configuring your own - I've done this for a number of projects. Best part is the price - $59.99 US for a download, and you can also obtain Canadian and US topos for a very reasonable price from the MacGPS Pro site. I've run it on everything from Mac OS 10.3 to 10.6. I highly recommend it.