Once we get down to the level of neighborhoods, below the township scale, finding GIS data can be very tricky. Neighborhoods usually aren't officially recognized, at least not by any body large enough or interested enough in creating accurate GIS data descriptions beyond "the houses between East Smith Street and the river."
Online real estate firm Zillow has created this collection of 7000 neighborhood shapefiles and associated geodata. They're under a CC BY SA license so you can use them on anything as long as you credit Zillow and share them along.
This is now my first stop when searching for basic boundary shapefiles (state, county, town), USGS raster data, and TIGER data (2003). I'm not sure who runs this site but the simplicity and open licensing are greatly appreciated!
The Library of the University of Arkansas has a very impressive collection of international and US geodata repository links - mostly geospatial data and attributes. I'll be poking around in here for hours, I think! A wide variety of topics and well-organized to boot.
A project of Columbia University, "SEDACs mission is to develop and operate applications that support the integration of socioeconomic and earth science data and to serve as an information gateway between the earth sciences and social sciences." In addition to a host of interactive mapplications, they host a large number of academic data sets on a wide variety of topics.
Sometimes you get tired of "armchair mapping" -- working exclusively with digital tools from the comfort of your home or office. Sometimes you want to go ride bikes through the Yosemite Valley, marveling at the waterfalls and cliff faces and rushing rivers. Sometimes your friend Alan gets a new Garmin Oregon 400t handheld GPS unit and you want to go play with it. Sometimes all three of these things happen on the same day PLUS it's your birthday so you can totally feel justified to take the day off to go do it as long as you blog about it later!