I've found that GlobeXplorer has the most recent images for my area.
However it is not free. I did a one month subscription because I wanted to order a print of our property and frame it, and I wanted the most recent picture of our property - June 19, 2007. Other than that though I'd stick with the free sites.
The other sites, GoogleEarth, MSN Virtual Earth, etc all pretty much use the same USGS images. Of those services I prefer GoogleEarth, simply because it allows me to do an elevation exaggeration that really shows the difference in terrain height and provides other features not available in the others.