With Google's recent crackdown on Web sites the sell PageRank--which really means selling links--a new era has begun for backlink building. In Google's eyes, links coming into a given Web site from external, quality sites increases that site's PageRank, and therefore its standing in the search engine's eyes. Until recently, there were many sites that had quality in Google's eyes (in other words, they had great PageRank) and also sold links. Anyone could get a piece of that good PR for a price.
Google is now actively lowering the PageRank of sites that deal in that sort of business, especially the larger, better known ones. These sites are often directories that require a fee to get a listing, and so far they are the hardest hit. I saw this firsthand in a listing of directories I keep. Going through the top-level directories on the list, one well-known directory's PageRank had dropped from eight to three. Another had gone from six to a flat zero. Still others directories simply don't exist anymore, most likely closing up shop to cut losses and avoid pesky questions from paying customers.
So, are there still ways to buy links to improve PageRank? Well, first off, it has never been a practice that I've necessarily endorsed. At the same time, there were--until recently--certain directories that seemed trustworthy and respected by Google as represented by good PageRank scores. Now that many (dare I say most?) of those sites have lost that endorsement, it's hard to recommend those that remain--some of the lesser-known directories that have thus far been spared. Still, a good Google hunt will turn up paid-listing directories that have decent PageRank. Whether or not such entities are a sound investment or a "proper" moral choice remains to be seen.
The best bet is to stick with directories that are free. So far, if they don't charge for the listing and have good PageRank, they can generally be considered acceptable in Google's eyes. Make sure, though, that nofollow tags aren't in place. If they are, while the listing may generate a little traffic, the listing does little to help the PR of a site.
If you really want to invest money into links, another option might be buying sites instead of links. It's easy to spend several thousand dollars on a link-buying campaign, and with Web sites possessing decent PageRank selling for as little as $10,000, at some point buying such a site makes the best sense. But don't fall into the trap of turning it into a link farm! Google will notice sooner or later, and your investment will be nullified or greatly damaged. Instead, buy a site for which you might have legitimate need and use it in that regard, making links to your other sites a secondary, but useful, focus.