Likewise http://www.treehugger.com/ looks like it has genuine content - although it's obviously a little bit thin on the ground.
These guys are clearly a legitimate business selling a product (well, legitimate website anyway):
http://www.supremeheating.com.au/pool-heating-top/solar-pool...
Now compare those three sites, to these four:
http://www.diy-solar-power-for-homes.com/
http://www.energy4living.hottipsonly.com/solar-power-for-hom...
http://www.diysolarpower4home.com/
http://www.solarwindpowerguide.com/diy-solar-heating/
this one, too, links back to earth4energy - strikingly similar to "earth4living" above:
http://greenerhomediy.com/create-solar-electricity-build-diy...
then we have this incredibly reputable and highly respected forum whirlpool:
http://forums.whirlpool.net.au/archive/1539413
As a human I find it relatively easy to pick out the massively spammy sites amongst those results that are on the first page for the search "diy solar homes" - and I think that any time you were to get several sites that are similar in some set of ways to each other that rank for the same search term then they should be "de-ranked".
So for example, thousands of people use the same wordpress themes, you can't just say "they're all spammers". But if the top 5 results for a particular search all share some measurable characteristics, you could safely say "hmm there's something spammy going on here".
If you click anywhere on the page, you'll get a popup asking you to buy some book of theirs.
They must just have compiled huge lists of relatively specific search terms and have pages against each? But I would think this would be easy to identify and downrank..
It is a puzzle :/
Then there's the "what you need, when you need it" category, and then there's the "put your google search in the title even though the page has no relevant results" category (mostly software download sites - ie. try searching for "application to use nokia e72 with itunes" you get this site filebuzz in the top two spots that has a whole lot of ads and a bunch of crappy non-related downloads).
So if you add a "uniqueness" index - ie. find ways to "semantically tag" not just the textual content, but the layout and font choices etc. of particular sites, that will catch the blatant affiliate spam bullshit (diy solar homes, what you need when you need it etc.) and then just figure out a way to prevent those "file buzz" type sites from sticking my search term in the title tags (I actually have no idea how this is done) you'll eliminate like 95% of the spam.