Most of designers have an ability to notice sites with good design or at least site’s parts which they like. Probably some of them want to save those sites for future when they’ll need some sort of inspiration. There are different ways to do that.

First of all I must say that I like Firefox so both methods that I’ll describe depend on it.
The first method I used some time ago. The method is to make screenshots of sites and save them in different folders to navigation. Screen grab! plugin is very useful for that method. I was happy with the method before some time but after I find that not all sites could be saved. There also was a problem to organize them.
Few days earlier I found another way to solve the problem. It use another Firefox plugin - ScrapBook. The plugin makes everything I need. First of all it saves entire pages: imgs, css, html. There is chance that that won’t work in some situations but I haven’t found them yet. The plugin also can edit pages before saving. That can be used to save part of pages. And finnaly it has his own organize system which works fine for me. It looks similar to the browser bookmarks.
As for me, I would recommend the second plugin but probably the first one could also be useful in some situations.
p.s. I definitely would glad to hear what methods use another people. Feel free to comment here.