If you go to http://www.audiogalaxy.com for downloads you will often find when you download songs by any rock or metal band it will say 'Other listeners liked.......' Follow those links to the world or rock music......pick the songs at random like I do when searching new bands out.....okay you wont always get killer riffs but theres more to it than that.
I could recommend so many riffs so I'll give you the one thats stuck in my head right now......Rush's 'Spirit of the Radio'....thats a tough act to follow.
'There's no such thing as bad weather, there's only the wrong clothes...'