I agree that Black Belt is the grand-daddy of them all, but I think the older issues were way better then the current ones. There are some good articles in there, but way too many advertisements for my taste.
For what it is worth, Black Belt magazine has allowed Google to scan all of their back issues up to (as I recall 2004) and place them online for free.
http://books.google.com/books/serial/ISSN:02773066?rview=0
Fully searchable too; so you can find articles easily. Google has done a great community service with Google Books, and many magazines (not just Black Belt) have done a great thing by allowing their magazines to be scanned and the contents made available for free to all. I give Black Belt great credit for doing this.