Available is a simple whois Applet that will probe any server on the Internet to tell you if it is available/up/running. It does a probe via ICMP and UDP echo port 7. IT also probes the home HTML page via port 80. As a side effect, it will tell you the IP address given the host name, or the host name given the IP address.

You can also run it as a standalone utility:
java -jar available.jar

# Often problems can be fixed simply by clicking the reload button on your browser.
# Make sure you have both JavaScript and Java enabled in your browser.
# This signed Java Applet needs 32-bit (not 64-bit) Java 1.5 or later. For best results use the latest 1.6.0_23. and a recent browser.
# Firefox 3.6 requires Java 1.6.0_10 or later.
# If you have Windows 7 64-bit and Internet Explorer 64-bit, you can use 64-bit Java.
# It will work under any operating system that supports Java e.g. W2K/XP/W2K3/Vista/W7-32/W7-64/Linux/OSX
# You should see the Applet above looking much like the screenshot. If you dont, the following should help you get it working:
# If you are using Microsoft Internet Explorer, try another browser. Seriously. Microsoft has taken great pains, over and over, to screw up Java and every other multi-platform standardisation.
# If you are using Internet Explorer 7 or 8, you must allow blocked content permission for Active X to run. This also gives permission to Java to run. Click the Information bar, and then click Allow blocked content. Unfortunately, this also allows dangerous ActiveX code to run. However, you must do this in order to get access to perfectly-safe Java Applets running in a sandbox. This is part of Microsofts war on Java. Dont put up with it! Use a different browser.
# For this Applet to work, you must click grant/accept to give it permission to talk to various servers on the net.
# Optionally, you may permanently install the Canadian Mind Products code-signing certificate so you dont have to grant each time.
# If the above Applet appears to freeze-up, click Alt-Esc repeatedly to check for any buried permission dialog box.
# If you have certificate troubles, check the installed certificates and remove or update any obsolete or suspected defective certificates. The only certificate used by this program is mindprodcert2011dsa.cer.
# Especially if this Applet has worked before, try clearing the browser cache and rebooting.
# To ensure your Java is up to date, check with Wassup. First, download it and run it as an application independent of your browser, then run it online as an Applet to add the complication of your browser.
# If the above Applet does not work, check the Java console for error messages.
# If the above Applet does not work, you might have better luck with the downloadable version.
# If you are using Mac OS X and would like an improved Look and Feel, download the QuaQua look & feel from randelshofer.ch/quaqua. UnZip the contained quaqua.jar and install it in ~/Library/Java/Extensions or one of the other ext dirs.
# If you still cant get the program working click HELP for more detail.