Wanted: Good Free Rules (Help Appreciated)

It's been suggested that we look at free rules.  

Now, I don't personally do "proper" reviews on free rules (after all, they're free - you can try it yourself) but it might be interesting to collect good free rules (or simply free rules with good ideas/concepts) and publish them in small articles, with a brief description of each - similar in size/style to the book reviews I've been doing lately.

These will not be "in depth", carefully thought out and playtested reviews, but more "have a look at this - it looks interesting."  There are thousands of free rules out there, so I'm seeking help to sift the wheat from the chaff, so to speak.

HOW YOU CAN HELP

In the comments section  (note: you don't need to sign into anything to leave a comment) please note the following:

1. The name of the rules (duh) and a link to their location
2. The reason you think these are a good rules set (it might be only a particular mechanic)

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EXAMPLE

I nominate Sturmovik Commander.  
http://assaultpublishing.com/?page_id=820
http://freewargamesrules.wikia.com/wiki/Sturmovik_Commander
Reason: It's now a commercial game but early versions are still free. It's WW2 aerial combat, covering from the Fall of France to the skies over Berlin. It's by Marcin Gerkowicz who did the excellent Hind Commander helo gunship rules.  It's certainly more game than simulation, but it's extraordinarily thorough and well-produced for a freebie.  Has a bit of an Imperialis Aeronautica feel to it.

(OK, your comment doesn't need to be as wordy as mine, but you get the drift!)

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Yes, it's OK to plug your own free rules, provided they are complete, online, and freely available to download (i.e. no "email me at joebloggs@gmail.com and I'll send you my alpha rules" thanks) 

The blog readers here seem to be pretty energetic - and if I get the feedback I expect, we can make this a regular article. So you can find this post again if you are struck by inspiration, if you look at the labels on the right, there is a "Free Rules" link where you can find this post (and others with free rules in them.)


EDIT: Thanks to all the comments - keep them coming!
The first round up - #1 Sci Fi is here
The second round up - # 2 Historical is here