TechProfound

I’ve used all of these online resources over the years, at various times to try to troubleshoot or ensure things are setup and working the way that is desired.  I am not affiliated with any of them, just a user and providing links here.  If you know of other links or great tools like these, please add comments to this post.

Broadband Reports
http://www.dslreports.com/tools

I like the Speed Tests.
http://www.dslreports.com/stest

Also handy from a small screen mobile device, but currently only does download.
http://www.dslreports.com/mspeed

DNS Stuff
http://www.dnsstuff.com/

There is a wealth of good info there.  I often need to use the DNS lookup to make sure records are set correctly and have propagated correctly after changes have been made.

Also, the Email Test very often quickly helps me know why some recipients are having trouble receiving email from various sources – because they don’t use a secondary mail exchange that spools their mail until the primary comes back online.

There’s a lot of stuff there that helps figure out where some spam may have initiated from, where some obfuscated URL’s really are, etc.

PC Pitstop
http://www.pcpitstop.com

I like the tool they have that checks your machine against other similar machines.  You can do it anonymously, or you can get a free account and track your various tests over time.