Dotcom-monitor is a nice web resource which allows you to instantly get a snapshot of website availability and performance from nearly two dozen locations around the world. This is one of the few free online website performance test tools that give you global results from the end-user's perspective. When you are thinking about conducting a vulnerability assessment or penetration test for that matter there are a number of things you need to check and this has the resources to do it. It can also help with some basic enumeration and allows you to do some setup also for bespoke tests.
If you are a systems engineer, it's even more useful as uptime and performance can be scheduled and alerted on as required.
Web Servers - Availability and server performance information for your HTTP or HTTPS website. For web application scanning and tests this may be beneficial
DNS Trace - A DNS performance test that traverses the DNS tree starting at the root server level. This may provide details of resources not fully documented and provided by the organisation you wish to test
Network Trace - A remote traceroute test tool that will help you troubleshoot network and connection issues. This may also identify edge routers, switches, firewalls and other devices.
Network Latency - An instant snapshot of Internet Latency from nearly two dozen points around the globe. This may help determine if you have any issues testing exploits and conducting vulnerability scans
Email Servers - Test POP3, IMAP or SMTP mail server performance and availability. If you plan to use Social Engineering techniques or test services of your customer this may be a nice resource to consider alongside the following:
Email Blacklist Lookup - DNSBL lookup checks to see if your email server is listed in major spam blacklists.
There are some other utilities also on offer for those needing to carry out some administrative duties adn who are in charge of varied networks, servers and corresponding services