Here are a bunch of commands in stsadm that I find useful every now and then to manage my sites and site collections.
1. Backup a site collection: stsadm -o backup -url http://urlname -filename <filename>
2. Restore a backed up site collection: stsadm -o restore -url http://urlname -filename <filename>
3. Backup a single site: stsadm -o export -url http://urlname/sitename -filename <filename>
4. Restore a single site: stsadm -o import -url http://urlname/sitename -filename <filename>
(Note #3 and #4 require you to be careful of non-unique GUIDs if working with the same DB, there is a commandline parameter to force create new GUIDs)
Here are interesting problem solving solutions based on the above:
Move or Copy a site collecton - #1 followed by #2
Move or Copy a site - #3 followed by #4
Convert a site into a site collection - #3 followed by #4 on a top site URL
Convert a top level site collection into a site - #3 targeted to the top level URL followed by #4.
Break Apart a content database - Use stsadm -o createsiteinnewdb to create a top level URL first, and then use #3 and #4 on that target URL.