A Short, Sweet List of Free Scanning Services
To begin with, you get a sense of what a security scanning service can do for your networks and systems by trying out some free scans. Although there's always some truth to the old maxim that, "you get what you pay for," you may be surprised at how much you can learn from some fairly simple, straightforward tools.
My favorite place to start checking security posture is at Steve Gibson research, where you can use his free ShieldsUp and Probe My Ports (once you finish ShieldsUp, you'll get a button to access Probe My Ports) services. Especially for Windows users, his simple probes and reports not only let you know where you stand, but also provide a wealth of information about what to do to close various doors and windows (pun intended) that these scans may find open.
The folks at SecuritySpace will gladly sell you several types of security scans on a subscription basis. You can visit their site to perform a free scan and get a sample of what kinds of checks and services they can provide you. A relatively modest $10 fee will buy one shot at their most comprehensive scan; their various subscription offerings are by no means prohibitively expensive.
On your behalf, HackerWhacker takes the IGR principle to the max in its presentation, attitude, as well as in the services it offers. This scan showed me some open ports on my Internet gateway machine that I found surprising, including some that were related to information downloads to my cable modem that I didn't even know were in use! Click the Quick Scan link on their menu bar to try a free scan on your own system. Like SecuritySpace, HackerWhacker will also sell you scanning services.
DSL Reports, a site devoted primarily to the needs of digital subscriber line users, offers a reasonably thorough TCP and UDP port scan, as well as a NetBIOS scan (absolutely essential for Windows machines attached to the Internet). This was the most thorough and complete scan of any of these free tools, and is now on my favorites list.
Even venerable anti-virus vendor Symantec Corporation has a growing interest in security related matters. The company offers a free tool, Symantec Security Check, that performs both anti-virus and security scans on your systems and networks. This tool performs some checks most notably, on browser security and privacy settings that other free scanning services do not. It, too, is worth a visit.
A search on Yahoo with the string "free security scan" turns up over 100 responses, of which at least 30 or so appear worthy of further investigation. If you're interested in seeing more free security scans run against your machine, repeat this exercise and knock yourself out!