Mac App Store without admin password?
According to one poster in an Apple discussion forum, this may be possible in Lion:


  This is possible with Lion.
  
  There is a _appstore group. All the users that are in this group are
  allowed to install applications from the Mac App Store.   You can add
  a user to this group with the workgroup manager (install server
  admin tool first). Or by typing in this command on the terminal.

dseditgroup -o edit -a shortusername -t user _appstore



Unfortunately, this approach won't work in Snow Leopard or earlier versions of OS X. There also doesn't seem to be an option to install apps in a directory other than /Applications.