Yeah, i like to do that. I've added a lead to a few songs just to see if it would sound cool. Its really fun doing that cause sometimes it sounds good and you can make a song even better than it already is.

The songs i enjoy playing the most are:

Awesome:
mithotyn-shadows of the past, king of the distant forest, hail me, embraced by frost, the old rover... and more to come
deicide-serpants of the light
god dethroned-the iconoclast deathride
fun/easy:
COF:funeral in carpathia, cthulu dawn (i like their guitar...)
fear factory: ressurection
i usually dont play a whole song, but for those i do.