There are a variety of reasons why (updated a DNS record, issues with resolver, etc) but sometimes a good diagnostic step is to clear out the dns cache.
Each operating system (and even versions / distributions in some cases) have a different method
> sudo discoveryutil udnsflushcaches
> dscacheutil -flushcache; sudo killall -HUP mDNSResponder
OSX 10.7 – 10.8
> sudo killall -HUP mDNSResponder
OSX 10.5 – 10.6
> sudo dscacheutil -flushcache
> ipconfig /flushdns
Linux - There are many different distros and versions here, but one of the following should work
> /etc/init.d/named restart
service nscd reload
sudo /etc/init.d/dnsmasq restart
service dnsmasq restart
Apple Discussions Support Forum - You can select the topic by OS, program, hardware or just type your problem into the search box.
CNET’s MacFixIt Feed - Tech blog CNET keeps tabs on all their OS X-related updates with the MacFixIt blog. Head here to search for updates, cool features and troubleshooting.
Quora - Apple Products - I’ve linked directly to the Apple-Products section, but really just punch in whatever you are having a problem with and search. Answers are provided by other users of the service, I don’t tend to use this all that much but others swear by it.
The MacRX Category on Cult of Mac - Cult of Mac keeps it simple: All Mac, all the time. Follow their troubleshooting-specific MacRX category to get updates on everything from good Mac maintenance and un-deleting files to how to fix start-up problems and get your Mac running fast.
Macworld Forums - Typical forum site with categories, search, etc.
macnn - Typical forum site with categories, search, etc.
reddit list of apps - large list of apps done by reddit users
fluid - Make standalone apps out of websites
opensource mac - Free Mac software
vienna - desktop rss reader
gimp on osx
OSX Daily Tips
SuperDuper - Drive Cloning software
Onyx - System diags
Disk Inventory Deluxe - shows space used
Reset Password on 10.4, 10.5 and 10.6
Reset Password on 10.7 and 10.8
Recover saved WIFI password
Adjusting Keyboard Backlight
Refurb.Me - Lets you track refurb deals on the Apple store
OS X Server Caching
Thread about OSX Server
Hidden Mavericks Pictures
Apple Video adapters and cables
This copy of the Install OS X Mavericks application can’t be verified Fix - http://blog.mconserv.net/2013/10/install-os-x-mavericks-application-cant.html