I use branches a lot but occasionally I forget to delete the branch after I merge it into master and then when I look at my list of branches I get a list of branches that I can't remember:

user@server:/var/www$ git branch 
  feature-calendarupdate 
  feature-calltype-filters 
  feature-multiselect 
  feature-newbilling 
  fixduplicatefields 
  fixresults 
* master 
  problemtracking

If you want a quick way to determine which branches have already been merged into the current branch (I usually run this at master because then I know they're "finished" branches) you can run git branch --merged to get just the merged branches.

user@server:/var/www$ git branch --merged 
  feature-calendarupdate 
  feature-multiselect 
  feature-newbilling 
  fixresults
* master
  problemtracking

Then you can git branch -d blah to your heart's content.

You can also run git branch --no-merged to see the branches that haven't been merged yet.