If you're using Mozilla (or I think it'll work on the brand new Netscape 6.1 beta), create a bookmark, set its URL to http://www.livejournal.com/users/%s. Then in the bookmark properties, set its keyword to lj, then you can type "lj foo" in the location bar, and get transported directly to foo's journal.

I think it's possible to do something similar (or possibly identical) in Internet Explorer, but I don't really use it any more.