Here is a quick rundown of what's played right now, if that helps. Neutral cards:
Edit: wait, I forgot souldbond stuff, I'll put it on bottom.
Other cards than these are rarely/never seen. Murlocks are only seen in dedicated deck, which ain't all that great.
1 drops
Argent Squire - 1 drop of choice of most decks, as it trades good with two 2/1s, and if you buff it it makes even better trades.
Leper Gnome - It's basically 2-4 damage for 1 mana, sometimes a lil more, usually aggressive deck run it.
Young Priestess - Zoo staple, any other agro deck that benefits from the effect.
Abusive Sargent - Zoo and similar very agressive deck that like good trades.
Shieldbearer - Zoo Staple.
Southsee Deckhand - Tempo Rogue.
Worgen Inflatrator - rarely seen, some players prefer this drop to other ones in agro decks. Like I saw some Shockadins that play it.
Elven Archer - anti-aggro tech.
Stonetusk - Hunter only.
2 Drops -
Thalnos - Anyone who can use +1 spellpower, which is many decks.
Ancent Watcher - Control decks, it comes in package with Defenders of Argust and..
Sunfury Protector - Control, see above.
Knife Juggler - Staple, any decks that can put out a lot of minions. Bonus points when it kills Leeroy before he attacks.
Dire Wolf Alpha - Aggro staple.
Wild Pyro - any deck that can cheat with it, i.e. Paladin/Druid/Priest control.
Loot Hoarder - Value creature, anybody who needs early body and draw power.
Faerie Dragon - when you need a 2 drop that doesn't die to early removal. Quite Decent.
Amani Berserker - Usually 1 of in Agro decks, it an be awkward to play against.
Ironbeak Owl - chapest neutral silence + trades. Played usually in deck that have to remove taunters.
Doomsayer - Freeze Mage/other low creature count controls.
Acidic - rarely, only those decks that get totally ripped by weaposn.
Bluegill - Aggro
Kobold Geomancer - if you see somebody play it, it's budget replacement for Thalnos.
Crocolisk - Midrange hunters. Trades favorable with 2/1s.
3 Drops
Harvest Golem - Super-value, anybody who can fit it.
Acoyle of Pain - those who can abuse it and need draw, i.e. Mage/Warrior Control.
Earthen Farseer - any deck that needs to surive a bit more, control and such. Mircle Rogue too.
Big Game Hunter - deck that don't have real removal (like druids) and need to get rid of big guys.
Arcane Golem - Finisher, if Leeroy is not nuff.
Injured Blademaster - Priests. Control ones.
Blood Knight - Meta-card, used when you think he has a lot of shields to steal.
Scarlet Crusader - Zoo, aggro.
Coldlight - some tempo Rogues, burn Mages, etc..
Mukla - some Hunters, some tempo Rogues..
Demolisher - Incrisingly popular in Druids.
Wolf Rider - aggro.
Shattered Sun - Zoo and zoo like.
4 Drops:
Leeroy - #1 favourite finisher, aggro, but basically anyone. Rogue cheats with it.
Defender of Argus - anybody who can fit it in, actually.
Twilight Drake - Handlock.
Dark Iron Dwarf - Zoo, aggro.
Violet Teacher - tokens.
Yeti - Those than need solid mid-game drops. Druid sometimes.
Sen'jin - same, but you also want to slow down aggro.
5 Drops -
Azure Drake - Everybody can use spell damage, good body and +1 card. Elite drop. Everybody play it.
Faceless - Usually control and other decks that can make huge minions and than copy them.
Gadgetan - Role player, Rogues mostly.
Harrison Jones - Control Warrior, I guess.
Kodo/Tiger - few midrange hunters, but those are dying out.
6 Drops -
Cairne - Elite drop. Midrange mostly.
Argent Commander - If you need to utility/burst.
Black Knight - see Big game hunter.
Sylvanas - some still run it, control decks.
Sunwalker - Hard to remove so he is played occasionally in slower decks, Midrange/control.
7+ Drops -
Geddon - control warrior.
Rag/Ysera - control finisher, Rag is more popular.
Alextrasza - Control. Freeze Mage/Warrior mostly.
Mountain/Molten Giant - Handlock.
Somebody else can do class cards.