I found inSSIDer (snazzy name) tremendously helpful. And it's free.
Looking at a very weak signal with my USB ærial pointing this way and that, got me the best signal possible through four old redbrick Essex walls. Yep, it seemed better in a straight line than pointing at the window etc.
It shows both common channel ranges.
The software tells me my channel, other local channels being used, and their relative strengths. They show WEP etc., of each local internet user, oh, and their provider.
It does not function when hard-wired however, which is a shame.