I did this last year, and thought it might be fun to do it again this year. To make this less of an exercise in navel gazing, I've linked to a lot of the things mentioned, including some freebies :
Best Set of Rules Acquired (and Played): Tactical Combat WWII Rules This is probably my favorite rule set of 2013 - it's free and it fills a needed spot in my WW2 rules - something for company level engagements. Runner up: Gunstorm! Another great free ruleset, ostensibly for sci-fi but I've used the NPC AI for troop movement in some WWII games and it's worked quite well.
Best Previously Acquired Rules that Got Use in 2013: Disposable Heroes. I wasn't sold on these at first. But, in the end, they won me over and I will probably play more in 2014.
Best Rule Purchase with an Eye Toward the Future: Pat Condray's Wargaming the Age of Marlborough Something about this thin little rule book excites me about gaming the period.
Best Non-Rules Book Purchase: Never Unprepared : The Complete Game Master's Guide to Session Prep
Best Free Non-Rules Download: Gamemastering by Brian Jamison. I got a lot from this freebie and have referred back to it many times. I've used the character background process with my players a few times and the results have been well worth it.
Favorite Gaming Accessory Purchase: B4: The Lost City
Purchase That I'm No Longer 100% Sure I Should Have Made: Reaper Bones II. I did drop out of the Vampire pledge level, but as I was locking in my final selection, I ended up getting paints and only a handful of minis. The paints will be more useful.
Favorite social game session (run) : The Dungeon of Akban The first social game I've ever run for people who weren't already in my close friend group.
Favorite social game session (played) : DCC at DragonCon
Favorite miniatures gaming session : Tied: Russian Tank Escort with Disposable Heroes/Coffin for Seven Brothers and Playing Gunstorm SciFi Skirmish and a Review