iStumbler is an application that shows nearby Wi-Fi networks along with lots of useful diagnostic information (such as signal strength and noise). It doesn't seem to have any built-in optimization tools, but it should get you started with some information.
List of features:
- Lists visible wireless networks with complete information.
- Inspector shows detailed information including a high-resolution graph, notes, extended properties and samples.
- Always displays the connected network, even if it's private.
- Graphically indicates network type and encryption status.
- Signal and noise graphed over time for the selected network.
- Connection monitor for connected network.
Wi-Fi menu extra
If you hold down ⌥ Option while clicking on the Wi-Fi menu item, you can see some more information about the Wi-Fi network you're connected to and others (hover over other networks to see information about them.)