The network analysis and diagnostics space is a big one. If you're looking for a general network layer sniffer, consider products from Network General (Distributed Sniffer) and Netscouts (nGenius line). Both are very popular solutions among IT pros. Solutions from Network Instruments (Gigabit Observed Suite System or GOSS) and Fluke Networks (OptiView) are less popular, but both have a very compelling appliance product that you might want to consider.
Two more application-layer products that we like are from Mercury Interactive and Coradiant. Mercury's Topaz product tests a site from the client perspective, giving you a look into your sites' availability and performance from the outside in. F5 has a partnership with them, and integrated our core BIG-IP Application Delivery devices to communicate with the Topaz solution. The result is Topaz can provide automated corrective measures by instructing the BIG-IP to route traffic away from affected resources. TrueSight from Coradiant is a bit different, focusing on detailed web and application performance information by analyzing real-time traffic flows. I think both solutions offer valuable perspectives that are worth exploring to get a good view into the application side of the house.
Also, don't forget that a great network resource in these situations is the load balancer. Yes, F5 provides solutions in this space, but folks often forget about the role load balancers play in network troubleshooting. As more and more traffic is encrypted, the load balancer has become the perfect place to take a closer look at what's happening to the traffic, or even mirror that traffic off a bank of deeper diagnostic devices like Coradiant.
This was first published in December 2005