Recover a deleted SharePoint site or site collection

Sharepoint 2007
The only way to recover a deleted site is through a database backup restore. Deleted sites and site collections do not go into the recycle bin.

Sharepoint 2010
Deleted sites and site collections go into the recycle bin, and can be restored from there.
http://www.mssharepointtips.com/tip.asp?id=1178

Sharepoint 2013
Deleted sites go into the recycle bin, and can be restored from there.
http://technet.microsoft.com/en-us/library/hh272540(v=office.15).aspx

You can also recover deleted site collections through PowerShell.
http://technet.microsoft.com/en-us/library/hh272537(v=office.15).aspx