User Steve has written a neat little program that, once installed and activated, will monitor your DSLR/BBR account for IMs and open a small window on your screen, along with an audible signal to tell you that you have IMs waiting. It works perfectly on Internet Explorer with Windows 95/98/ME/NT/2000/XP. It's very small, and the same code compiles on Linux and other UNIX platforms. Full source code is provided.

Steve's Disclaimer: This tool is a private effort and is not endorsed, authorized or otherwise commented on by DSL Reports management.

Learn more about it here.

For Firefox, see this.

For MAC users, there are two available:
Gomez 's BBRImChecker 2.0.1.app and
rjackson 's BBR IM Checker widget