Can't connect to a hotspot? Keep getting booted off the office Wi-Fi? Want to download the latest updates for your apps but can't because the iPhone armada has steamed in and pwn'd all the wireless?
You need Wi-Fi Analyzer. This amazingly handy app displays the signal strength of all Wi-Fi hotspots within range of your phone.
The channel graph display (right) measures signal strength against what the app calls a 'channel rating'. Channels are rated in terms of how busy the connection is - basically the higher the arc, the better the connection. So, in the pic to the right you'd want to connect to the red-coloured hotspot.
If you're already connected to a Wi-Fi hotspot you can use the app to compare your current signal strength to others nearby, by hitting the menu button and going to 'Set my AP'.
Unfortunately you can't connect to any hotspots within the app itself. You'll need to download a separate Wi-Fi Connecter Library app (also free) from the Market. This is an add-on for Wi-Fi analyzer and doesn't do anything on its own. [Cheers for the tip, Barry!]
Wi-Fi Analyzer is a free app which is ad-supported. If this bothers you can just turn the ads off in the settings. Or, if you're feeling charitble, you could always leave a donation.