I use the National Flight Data Center at http://nfdc.faa.gov
I had to write them a letter explaining why I wanted the data (http://fplan.sf.net) and they provided me with a login.
Now, every 56 days, they send me an email telling me there's an update available.
Bad news: they recently switched from a flat file to some unbelievably convoluted xml format that I haven't yet written a parser for.
Also, it comes with a disclaimer that the data is not approved for navigation. Given that I've already found two minor errors in it, just in the SF Bay area, that's understandable.
Edit: I forgot to mention that I keep a subset of the data on-line at SourceForge. I try to keep it up to date. It's at the Fplan project. These files are in the old "nav" format; documentation is included in the zip file.
If someone needs these files, they're freely available and I'm happy to make small additions to the data if anybody wants, since I already have the tools to parse the FAA data files. I'm also happy to share the tools if anybody wants.