The javascript I use is:
parent.Main_Frame.location = this.options[this.selectedIndex].value
I don't know if it's any better than Nick's, but it definitely works.
Be very careful when you use javascript, because major browsers have different ways to talk about html objects. So something that works in IE may not work in Netscape, and something that works in Netscape Navigator may not work in Netscape 6.
There are some good javascript web pages that talk about the differences.