no-ip.com provides different services. The so called "Free" service is
free. See http://www.no-ip.com/services/managed_dns/free_dynamic_dns.html

   On the other hand, the "Free" service is intended for "personal use
only". Not sure how that fits in with your site (which I believe you
mentioned makes a bit of money). You might want to read the contract
carefully to see what is or isn't allowed.

   I used to use the free service, but eventually I just bought a domain
name and a static IP.

   - Oliver

No-ip.com is free, I've been using it at work for ages and it's still free.
Also dyndns.org is free. I actually recommend dyndns.org, because they
provide you with a free download called noipDuc (or similar) that runs
on your machine and updates their DNS servers whenever your external IP
changes.

Also, my wireless router (Linksys WRT54G) has a feature in the admin web
pages to allow auto updating of your dyndns address, so I don't even
have to run noipDuc on my desktop machine any more.

I'd seriously avoid doing this in your own code - I appreciate it works,
but it seems inelegant, when dyndns.org works so well and is free.

I have created another solution that works pretty well, and it is entirely
self-contained (ie, no third-party software or fees).

I have a small program running on my home server (Sun Netra).  It's actually
two threads: one is a mini-server, which listens on a local port; the other
will access that port every few minutes, at my last known IP address, asking
for a magic number.  If the magic number is wrong, or there's no response
from the mini-server, then the IP must have changed, so the program will
then access a CGI script on my remote (paid-for) website.  That CGI script
will extract my home IP from the request header, and save it to a file.  It
will also return the IP address back to my home server, where it will be
saved to my "last known IP" file.  So now my home IP is saved at both ends.

It gets better.  I have another CGI script on my remote web-site, which acts
as another mini-server.  When this one is accessed, it will open a
connection to my home server (it has the IP address, remember!), and will
pass the query portion of the request to the home server, where it will be
passed to Tomcat, which creates HTML from a mixture of JSP and Servlets.
And because the home server also knows the home IP, it can plug that IP
address into the newly-created pages.  What this means in practice is that I
can create content which contains references to images (for example) that
are on my home machine.  The address bar in the browser still points at my
remote website, while some (or all) of the content is here at home.  And
even if my IP changes, the generated pages will always contain the correct
IP.