Quick and dirty you could create a php file on a remote machine that captures and echos the ip of visitors. Then just browse to that site anytime you need to know what your ip is.
ex:
echo "$HTTP_SERVER_VARS['REMOTE_ADDR']";
Alternately you could download various keep-alive programs. I think most of them have a mechanism to retrieve your external routable ip address.
Also alternately consider a dynamic DNS service such as www.TZO.com (costs) or www.whyi.org (free)
they keep track of your IP for you so you could just browse to:
yourname.yi.org:8080 for example. Whenever you want to.