A lot of motorcycles, like my '05 vstar 650, have no power drain when the key is turned off. There is no key fob or security system drawing power.
If your bike is simple like this, then you should be able to park the bike over the winter, and just check the battery voltage once a month like Mrdub said. If its low bump it up with a charger till it hits 13V and you are good for another month.
I was going to buy a battery tender, but they were $30, and I only paid $25 for my MC battery (got it online). I could make one with a 12V wall adapter (500mA) by putting a linear 12V regulator on the output, so it would never push the battery up over 12V, but then I have to plug it in, have a wire running across the garage all winter... Its easier to just leave the side cover off the 650 and spot check the voltage once a month.
If your bike has a bunch of stuff that is drawing power when the key is 'off', then yes, you definitely need a tender. Lead acid batteries MUST be recharged right after they are used, or the battery will quickly fail. If it slowly drains down over the winter, the battery is ruined.