First off you'll need to know where your wireless connection's boundaries are. Along with this, you'll want to know if other wireless networks overlap yours. In an urban or multi-business environment, neighborhoods where homes are close together, and in apartments you're almost certainly able to find other access points. You or a user might associate with another access point (if it's open) without even noticing that it happened. Suddenly data is being transmitted in the clear and your wireless computer is accessible from that network!
NetStumbler is a Windows program that finds open access points. I like it because it's fast and easy, and because there's a Windows CE version for my handheld.
Kismet is a very full featured 802.11 detecting, sniffing and decrypting tool. This is the premiere tool for wardriving. Primarily for *nix environments, Windows support is possible but very limited. Sniffing logs support WireShark & TCPDump formats if you want to work with your usual sniffer program. Real-time WEP cracking and decoding is possible, as is subnet mapping and GPS support. If you want to discover every wi-fi network and access point in your area, this is the tool to use.
Granted, this is more of a hacking tool than a securing tool. What's important about Aircrack is its speed. This tool will show you just how insecure WEP security is, and can give you an idea how easy it would be for a hacker to crack your WEP or WPA wireless security. Once you've seen that, you'll want to implement additional security for your wireless connections!
SSH is not specifically a wi-fi tool, of course. Many network devices have telnet access as an option for management. You really should never be using telnet in today's networking environments. Why are you? Most modern devices support SSH as well. If you use Linux, OSX or other *nix implementations you should be using SSH for all your remote console-based administration. For applications that don't support SSH, you can often use Stunnel, to tunnel the application's TCP connection through an SSH connection.
What if you or your users are at a remote, unsecured hot spot while travelling? Your best security at home won't help there. What to do? Well, configuing remote connections to use a point-to-point VPN is ideal. That makes this an important tool not just for wireless security, but all remote security. A secure wireless connection with an IPSec VPN running on it adds another layer of security and encryption. A VPN initiated by the user when accessing company (or home) resources remotely ensures that if wireless packets are sniffed, they're encrypted. VPN encryption can be configured to be very strong. Using PKI or other two-factor authentication can virtually eliminate the risks of man-in-the-middle attacks.
Get To It!
For businesses, having your wireless users set up for authentication ensures several things. First, an outsider, hacker, or whomever can't connect to your wireless network without effort. Second, if you implement two-factor security for authentication such as USB tokens, even if a user id and password were stolen, they can't be used without the token. These types of connections are more difficult to crack and use as well. Granted, USB tokens, hardware keys, or smart cards aren't free--but seriously consider adding them if you need the additional security. PKI implementations can be free, but can also take more effort to implement and integrate. The tools above will get you started evaluating and adding to your wireless security.