Minecon capes can only be obtained by attending Minecon. The exception to this is if you have Pocket Editon or Console Edition where everyone can get the capes download after Minecon. The caveat to that though is that the cape is only wearable by the Steve or Alex skin.
OptiFine capes can be seen by anyone using OptiFine. You will need to check their website to see what you have to do to get one.
There is a mod out there (I saw it on OMGCraft) that will allow you to use a cape of your own design. However, that cape is also only visible to anyone who is using the mod.