Sometimes when you're developing a website that you know will be served on 'https' it's nice to be able to test using 'https' to make sure things work as expected. You could go the route of creating your own certificate using something like
makecert -r -pe -n "CN="%computername% -b 01/01/2000 -e 01/01/2036 -eku 220.127.116.11.18.104.22.168.1 -ss my -sr localMachine -sky exchange -sp "Microsoft RSA SChannel Cryptographic Provider" -sy 12
but then you have to deal with the pesky messages that let you know that it's NOT a valid certificate, and/or adjust your browsers settings for your site etc. There is an easier approach to this. Many certificate authorities are now offering trial certificates. All you have to do is just give them a little bit of info about yourself (I guess you could use someone elses though) and you're good to go for a period of time.
I recently received a certificate from Thawte that will last me 90 days. This is plenty long enough for dev/testing of our site on SSL and I have a valid certificate that wont cause my browser to yell at me. Thawte was just the one that I happened to go through you can also get a trial from VeriSign although the trial isn't as long.