The Android Market recently added a new statistics page for app developers, giving useful information about each app such as downloads, which devices and languages are the most popular for your app and so on.
Unfortunately the download numbers are embedded into a Flash chart, with no easy way to extract the raw data for further analysis.
I whipped up a quick bookmarklet to fix that. Drag this to your bookmarks bar, then run this on the stats page, and you'll get a simple tab-separated list. The first column is a timestamp, the second column is the cumulative number of downloads. You can paste this into a spreadsheet easily.
Here's the bookmarklet: Extract data
(I used the Bookmarkleter to convert the simple JS code into a bookmarklet)
Of course there are third party services who will grab the data for you, but if you just want access to some quick data, this should do the trick.