I have most (perhaps all) of the free cards listed on my site below. Amtrak gives United miles. The hotel cards, expecially Hilton, give miles to many accounts, but not very generously. I think you are best off getting a card that gives miles to one specific airline, getting the miles you need for it, then moving to another card for another airline. The Delta Amex card can be had for free for one year and gives 10000 up front miles, for example.

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The Personal Travel Experience of Gary Steiger - including how to get free frequent flyer miles on the web.
http://home.earthlink.net/~pgary/TravelFrame.html