A WebmasterWorld thread has a short discussion around a basic but solid link building tip. Basically the strategy is to find third-party web sites that link out to sites, but those links are simply linking to pages that no longer exist. Then politely email the site owner informing them of the broken link and suggest that you know of a resource that can replace the broken link.
The tip here is credited to Martinibuster, posted by wheel. I should note, Martinibuster has given this tip for almost as long as I have known him. But let me quote Wheel's interpretation of it:
Here's what I'm doing over the next month.
1) I'm going looking in the really old 2000 era directories specific to my niche. 2) I'm going to run Xenu link sleuth on the directories to find broken links (again, these will be links to sites in my niche). i.e. I'll build a list of defunct sites in my niche. 3) I'm going to find sites that link TOO those defunct sites, i.e. busted links on other sites. 4) I'm going link begging. Basically saying "hey, you got a busted link. And while you're fixing your web page, howzabout hookin' me up?'.
Worth a shot and you are doing the web a favor. :)
Forum discussion at WebmasterWorld.