Tweak tester
The tweak tester analyses a sample of data and can tell you, among other things, what your MTU and RWIN are and whether or not you are blocking PMTUD. Changing these settings can sometimes speed up your connection.

Antivirus tester
This tools uses the harmless Eicar test string to test your Antivirus protection. The Eicar string of letters and symbols which is recognised by most AV products as a virus. The string was specially designed for AV testing.

LineMonitor
This tool pings your IP address twice every second. It is very good for picking up low level faults which can be the cause of disconnections. LineMonitor can be set to email you when your connection fails.

nScan
nScan is a security / port scanner powered by Nmap scanner. Nmap is one of the best (if not the best) security scanner available. NB: do NOT use this scanner unless you are authorised to do so by the system and network administrator(s).

Site Up?
This site will atempt to get web pages from sites. Very useful if you can't access a site and you need to know whether it's you or them.

Traceroute
Run traceroutes using tcptraceroute and traceroute to various targets.

Web server test
Tests for a web server running on your connection.

POP3 server test
Tests for a POP3 running on your connection.

SMTP server test
Tests for an SMTP server running on your connection.

FTP server test
Tests for an FTP server running on your connection.

SiteMonitor
This tools checks the availabilty of a site every two minutes and can be set to email you if it fails.

Ping me
This test checks to see whether you are pingable.

My IP
Shows your internet address and if you are using a proxy server.

Lookup tools
These tools allow you to find informtaion about things like domain names and IP addresses. For example, where a domain name points to, who owns the domain name, who owns the IP address it points to, etc.