I use the free version of Acrylic on a windows laptop - it works really well. It misses some advanced features of the full version but is more than sufficient for scanning channels etc.
For the Mac, I use NetSpot and it works great. It will show all the nerby access points, channels, strength, etc. The free version is fine. It's also cool to "Map" your wifi with the paid version.
Also, on a Mac, there are some pretty decent tools built-in!
Hold down the "option" key, and then click on the WiFi menu item. You'll see addtional items under the menu. Click "Open Wireless Diagnostics..." Then, with Wireless Diagnostics open, click on the "Window" menu to find various other tools. I have found the "Scan" tool to be particularly useful.
