There are tons of options: Attrition or Blood Rites , while requiring to pay mana, would kill your opponents creatures (or the opponent himself in the case of the rites).
If you need something that doesn't require mana to activate you could try Bloodthrone Vampire , Carrion Feeder or Devouring Swarm .
Hell's Caretaker can be an interesting choice, because it gets creatures back when sacrificing others, but can be activated only once per turn.
Lyzolda, the Blood Witch can be sweet, drawing you cards when sacrificing black creatures or dealing damage when sacrificing red ones.
There are lots more, here you have Magiccards search of creatures that have an activated ability which requires to sacrifice a creature and are black, blue or red.
boardgames.stackexchange.com/users/2314/mornaner
http://gatherer.wizards.com/Pages/Search/Default.aspx?action=advanced&color=+![G]+![W]&text=+[%22sacrifice%20a%20creature:%22] Teach a man to fish.
The Gatherer is your friend.
Once you know how to use it, you can find many of your own answers.
You might want to examine the Advanced Search to narrow your search, or the Help Page to get an idea how to search.
Explanation of the URL variables above
('?' starts the query, '&' separates variables, Left Hand Side (LHS) is the variable name, RHS is the value of the variable): action
=advanced (gives responses for a greater range of variables) color=+![G]+![W]
(card mana cost does NOT ('!') contain Green AND does NOT contain White mana) text=[%22sacrifice%20a%20creature:%22]
(contains the exact RULES TEXT ('"' is %22, space is %20.
URLs cannot contain these special characters, so they must be encoded)
"sacrifice a creature:" You can type your own search examples on the Advanced page, and then examine the URL to see exactly how your query is posted to the database.
Technically, this search result would possibly show us cards that don't match the color identity of the Commander, if a card existed that contained a [G] or [W] activated ability, but not a whole lot of these exist.
So just for fun, try replacing the RULES TEXT with this search strings.
text=+![B]+[%22Sacrifice%20a%20creature:%22]
This will return all sacrifice a creature activated abilities on cards that don't contain Black Mana symbols on the card.
You will note that the botto of page 0 is Quagmire Druid which doesn't match our Commander's color identity.
boardgames.stackexchange.com/users/1873/user1873
Although I generally shy away from suggesting a single card, in this case Braids, Cabal Minion fits the bill so perfectly that it's tough to ignore.
boardgames.stackexchange.com/users/1583/Stephen
You should have a look at cards that make each player sacrifice creatures/permanents.
Examples:
Pox, smallpox, smokestack, innocent blood, fleshbag marauder.
boardgames.stackexchange.com/users/1784/AndSoYouCode
http://gatherer.wizards.com/Pages/Search/Default.aspx?text=+[%22sacrifice%20a%20creature%22]
Without wishing to suggest specific decks for you, the above link should at least give you plenty of ideas to get your creative juices going!
boardgames.stackexchange.com/users/436/thesunneversets
Most Thrax decks I've seen use Grave Pact or Butcher of Malakir .
These are good "rattlesnake" cards that also greatly magnify the value of anything you feed to a sacrifice outlet.
Attrition is also worth mentioning, since it's such a cheap outlet with one of the best beneficial effects.
Consider Malfegor and Death Cloud if you want additional bulk effects of this type.
Sacrificing your own dudes for mana is also pretty awesome because you can often turn that mana into more dudes to sacrifice.
Phyrexian Altar and Ashnod's Altar serve as the basis for many token-sacrifice engines.
Skirk Prospector can do the same for Goblins, as can Basal Sliver for Slivers.
Cards that create Eldrazi Spawn -- in particular, repeating ones like Pawn of Ulamog and Rapacious One -- are also good for giving you a bit of extra mana while Thrax has something to munch on.
In my Sheoldred reanimator deck, I actually use fatties as sacrifice victims, feeding them to Soldevi Adnate , Miren, the Moaning
Well , and Xathrid Demon .
Minion Reflector is an excellent way to get double value out of ETB triggers.
The tokens it creates have a built-in self-sacrifice effect, but you might as well feed them to something else.
boardgames.stackexchange.com/users/1910/Alex P
Found a few that I have somewhere in my collection that look like good options: Carrion Feeder Phyrexian Ghoul
Thanks for all the suggestions
there are far more cards suited enabling sacrifices than I thought.
boardgames.stackexchange.com/users/2182/ThunderHorse
I'm building my first EDH deck and have chosen  Thraximundar  (Grixis) as my general.  My deck has a lot of synergy centered around sacrificing creatures.  I find it easy to benefit from sacrificing and have plenty of targets for sacrificing but I need more ways of enabling the sacrifice triggers.    So far I have  Viscera Seer , not much to go on.
Sacrifice enablers for EDH
boardgames.stackexchange.com
<magic-the-gathering><deck-building><mtg-commander>
boardgames.stackexchange.com/questions/6281
boardgames.stackexchange.com/users/2182/ThunderHorse
You could try Bloodthrone Vampire, Carrion Feeder or Devouring Swarm. Hell's Caretaker can also be an interesting choice, as well as Lyzolda. There are many options to choose from e.g.  Attrition , Braids, Cabal Minion or  Blood Rites. Or if you need something that doesn't require mana to activate you could try  Bloodthrone Vampire ,  Carrion Feeder  or  Devouring Swarm .  Hell's Caretaker gets creatures back when sacrificing others, but can be activated only once per turn.   Lyzolda, the Blood Witch  can draw you cards when sacrificing black creatures. Look at cards that make each player sacrifice creatures.  Malfegor  and  Death Cloud  are good for additional bulk effects.  And Phyrexian Ghoul  is also useful.
There are many options to choose from e.g.  Attrition , Braids, Cabal Minion or  Blood Rites. Or if you need something that doesn't require mana to activate you could try  Bloodthrone Vampire ,  Carrion Feeder  or  Devouring Swarm .  Hell's Caretaker gets creatures back when sacrificing others, but can be activated only once per turn.   Lyzolda, the Blood Witch  can draw you cards when sacrificing black creatures. Look at cards that make each player sacrifice creatures.   Malfegor  and  Death Cloud  are good for additional bulk effects.  And Phyrexian Ghoul  is also useful.