One of the annoying things with VS 2005 is that you have to first run your ASP.NET pages before you can set JavaScript breakpoints in them in the debugger.  

VS 2008 makes this much better by adding new support that allows you to set client-side JavaScript breakpoints directly within your server-side .aspx and .master source files:

Figure 1

When you set a breakpoint in your .aspx page like above, VS 2008 will automatically map the breakpoint location to the dynamically generated client HTML that runs in the browser when the page is later executed:

Figure 2

If you add/remove/update the breakpoint locations in the running HTML document, VS 2008 is also now smart enough to perform the reverse mapping and update the breakpoint in the original .aspx or .master source file on the server.  This makes it much easier to get into a nice edit/debug/edit/debug flow as you are iterating on your applications.

Best of all, you can now set both client-side JavaScript breakpoints and VB/C# server-side breakpoints at the same time (even in the same page) and use a single debugger to step through both the server-side and client-side code in a single debug session (which is extremely useful for any AJAX heavy application).

Any JavaScript breakpoints you set will also now by default be saved by VS 2008 when you close the project/solution.  When you open up the project again, the previous locations you set the breakpoints on will still have them enabled.