If I may jump in and deprive Gail of a point...............(She's got plenty)

If your database data file is obviously way over sized and you you know you will never need the file that size and besides you need the disk space, then yes, as Tim says, that is an occasion when you can shrink the file. Of course if you do, don't shrink the file down to its minimum possible and immediately follow it with a reindex.

An option is to use the truncateonly option of dbcc shrinkfile (don't use shrink database). This does not move data about and just trims the file to the last used extent. Depending where this extent is it might recover no space, about what you wanted, or more than you wanted. If more than you wanted increase the file again manually to the desired size. For a large database this method can be much quicker and less intrusive.