I'm not exactly a programmer so if I give a direction that doesn't really make sense forgive me. I did not write this and I don't really know who did, but whoever they are they're pretty smart.

First, you must get perl. I downloaded Strawberry Perl, it works pretty well.

Tools you will need to unzip into one folder. http://www.mediafire.com/?nwui2mqj2mk

I make these into .bat extensions so that I can download many tags automatically, but you can use cmd.exe as well.

1. Navigate into the folder containing the tools using command prompt. (cd C:\booru is my location)
2. The syntax you will use is "perl booru.pl [booru location you will be using, in this case danbooru] [tag, can be more than one]"

Take a look at the example enclosed. the line "perl booru.pl danbooru hakurei_reimu -f={md5}" will download all images with the hakurei_reimu tag and name them with their md5 hashes. There are more modifiers you can add to make your life easier.

-f= (filename template, I just leave these to {md5})
-l=NUMBER (limits downloads to a certain number)
-s=NUMBER (downloads from a post # forward)
-r=NUMBER (Downloads based on rating. 1=safe 2=questionable 3=explicit)
-p= (sets path to save the files in instead of the current directory. a real lifesaver when you start to put this stuff in batch files. Remember to use quotes if you have any spaces in your path)
-u=username -w=password = These should be put in to avoid the error of "couldn't get at line" xxx or whatever.

example:
perl booru.pl danbooru ibuki_fuuko -r=3 -f=${md5} -p="C:\Pictures\Fuuko"

Will download all ibkui_fuuko tags that are rated explicit from danbooru and name them with their md5 hashes and place them in the C:\Pictures\Fuuko directory.

There is some documentation included with this, although its not very user friendly. I don't know everything about this program, but if you want to ask me questions you can PM me.