Network monitoring is the logical next step after getting a good firewall and virus scanner for your computer. It allows you to specify specific rules and programs which should or shouldn't be allowed to connect to the Internet. This puts more of your computer under you control, ensuring that no programs send data that they aren't supposed to.
Most anti-virus software nowadays have a network monitoring function. For Kaspersky, it's called Network Monitor. Windows 7/Vista has got a built-in feature that's similar, but not as effective. It's called Resource Monitor and it resembles the Task Manager. You can start it by simply typing Resource Monitor in the Start Menu.
1. Little Snitch Another popular network monitor for Macs is Little Snitch. Little Snitch will allow you to intercept unwanted or undesired network connections and decide how to address these. You can allow or deny these connections. Alternatively, you can define broad rules to allow programs like browsers unrestricted access. This way, it can also detect activity related to viruses, trojans, and other malware. You can try it for free, but the trial version only lets you use it for a three-hour period. You'll have to restart Little Snitch after that. Or you can just buy a license for around $40.
2. Peer Guardian Peer Guardian is freeware and you're encourage to donate to allow the developers to continue supporting it. You can find it here.
3. VirusBarrier Intego's VirusBarrier also includes their previous product NetBarrier, which works similarly. You can get the Express version at the Mac App Store for free but it's only the complete version of Internet Security Barrier that has all of the features. That version costs $80 over at the Intego store.
4. iStat Menus 3 iStat Menus 3 includes a network monitor, so it's another available option for Macs. It costs $16.
5. Kaspersky Anti-Virus Kaspersky has an integrated network monitor, much like some other anti-virus software.
6. Peer Guardian 2 Peer Guardian works for PCs as well.