If you've deleted a file in the ordinary way (not overwritten), you may be able to recover some or all of it. This isn't guaranteed, and the process is sometimes complex and time-consuming, but this is what someone else could do who wanted to access your data.
The following are examples of programs that can recover, or undelete, files that have been deleted but not yet overwritten:
Boomerang Data Recovery. Boomerang can restore not only individual files but even entire partitions. It can also reconstruct some RAIDs and recover data from iPods. The product is licensed based on the volume of data you want to be able to recover — for example, with the least-expensive ($39.95) license, you can recover 100MB of data. (www.boomdrs.com/macosx, Home edition, $39.95 and up; Professional edition, $349.95 for a one-year license)
Data Rescue 3. This application can recover files from your computer's hard drive as well as from many digital cameras, iPods, and other storage devices. (http://prosofteng.com/products/data_rescue.php, $99)
File Recovery for Mac. Like Data Rescue 3, this program can recover files from your Mac's disk and numerous other storage devices in cases where data has been accidentally deleted, disk errors have occurred, or a disk has been inadvertently reformatted. (www.applexsoft.com/mac-file-recovery.html, $99.95)
FileSalvage. For recovering accidentally deleted files or those that have been made unreadable due to disk errors, FileSalvage offers ...