if your not going to use them over the winter, bring them into the house. If you are or just dont want to do that, the cheep $20 ones from harbor freight work just fine. got a 4wheeler i might use 1 or 2 times after October and just leave it plugged in all winter. quad starts right up no problem..

http://www.harborfreight.com/15-amp-three-stage-onboard-battery-charger-maintainer-99857.html

eta: quad is kept out in a old metal barn with zero insulation and common to have snow blow into it.

I've had several Harley batteries make it to the 10-11 year mark sitting in an ice cold garage.  I run a (4) bank Battery Tender and swear by it.

I had 4 of those on various lawnmowers and a a old snow plow. 2 failed in the first year. replaced them with what i listed above. last 2 are still working after 3 yrs total. i might have just got a bad pair. i liked them because there was no footprint when they were unhooked where the one i listed was stuck on the machine.

I got this one from Harbor Freight and it works just fine.   http://www.harborfreight.com/automatic-battery-float-charger-42292.html

I like the Black & Decker # BM3B one, you can hardwire it or just clamp it on. It's also waterproof and available at Walmart



   Keeps 6V & 12V battery systems at optimal capacity
   Fully Automatic high frequency charging
   Reverse polarity protection
   3 Ways to Connect: Batter clips, 12V DC Accessory Plug & Battery Ring Terminal
   AC Low voltage compensation ensures maximum performance when used with an extension cord


http://www.blackanddecker.com/products/power-tools/automotive/vehicle-maintenance/battery-maintainer--trickle-charger/bm3b

Yup, these are the industry standard.  Wouldn't recommend anything else.

These maintain and desulfat your battery http://www.batteryminders.com/ the battery Tender doesn't. None of them will charge a battery under 10V you need a old school charger for that.