Barack Obama has long-term ties to the far-left group ACORN. And, those ties continued at least as late as February 2008, when ACORN's Ohio branch worked with the Barack Obama campaign on a GOTV effort. See this job announcement from Mari Engelhardt, political director of Ohio ACORN:

GOTV for Obama! Ohio ACORN is doing a Get Out The Vote project with the OBAMA Campaign. Ohio ACORN is hiring canvassers to go door to door encouraging voters to vote for Barack Obama.

ACORN is hiring [in or for Cleveland, Columbus, Cincinnati, Dayton, Toledo]. Or email [address deleted] and your inquiry will be routed to the appropriate person in each of these cities. Intake and training will be held daily at local ACORN offices. Canvass begins on Wednesday Feb. 27th and will work through election day...

Please do not contact the Obama campaign directly regarding this post as they are not the organization doing the hiring and it will only distract their staff and volunteers from the other important work they are doing on behalf of Senator Obama.

And, from this 10/6/08 post:

Project Vote and ACORN... just wrapped up a voter registration drive targeting battleground states Obama needs to win the White House.

Though officially non-partisan, the focus of the ACORN/Project Vote voter drive was on groups leaning Democratic in the presidential contest: African American, young, Latino and low income earners. They are called "historically underrepresented in elections" in a press release issued by the group on Monday. Republicans would call these target groups Democrats.

ACORN/Project Vote ran voter registration operations in 21 states; included are the battlegrounds Colorado, Florida, Michigan (since move to Obama) Minnesota, Missouri, Nevada, New Mexico, North Carolina, Ohio, Pennsylvania and Wisconsin.

UPDATE: There's more on BHO's links to ACORN here, including where they get their money from, what they spend it on, and their history of submitting bogus voter applications and more:

Then there's Lake County, Indiana, which has already found more than 2,100 bogus applications among the 5,000 Acorn dumped right before the deadline. "All the signatures looked exactly the same," said Ruthann Hoagland, of the county election board. Bridgeport, Connecticut estimates about 20% of Acorn's registrations were faulty. As of July, the city of Houston had rejected or put on hold about 40% of the 27,000 registration cards submitted by Acorn...

Which brings us to Mr. Obama, who got his start as a Chicago "community organizer" at Acorn's side. In 1992 he led voter registration efforts as the director of Project Vote, which included Acorn. This past November, he lauded Acorn's leaders for being "smack dab in the middle" of that effort. Mr. Obama also served as a lawyer for Acorn in 1995, in a case against Illinois to increase access to the polls.

During his tenure on the board of Chicago's Woods Fund, that body funneled more than $200,000 to Acorn. More recently, the Obama campaign paid $832,000 to an Acorn affiliate. The campaign initially told the Federal Election Commission this money was for "staging, sound, lighting." It later admitted the cash was to get out the vote...

And, from this:

In the Indianapolis metro area, ACORN has been so successful that the number of registered voters now stands at 105 percent of the voting-age population.

And, this implies that the job application above was related to the $832,000:

An Obama spokesman said Federal Election Commission reports would be amended to show Citizens Services Inc. -- a subsidiary of ACORN -- worked in "get-out-the-vote" projects, instead of activities such as polling, advance work and staging major events as stated in FEC finance reports filed during the primary...

The Ohio primary was March 4. According to FEC records, the Obama campaign paid Citizens Services Inc. $832,598.29, from Feb. 25 to May 17.

A Trib analysis of campaign finance reports showed Obama paid CSI for services that stood out as unusual. For example, CSI received payments of $63,000 and $75,000 for advance work. Excluding the large payments to CSI, the average amount the Obama campaign spent with other organizations was $558.82 per check on more than 1,200 entries classified as advance work.

Citizens Services Inc. is headquartered at the same address as ACORN's national headquarters in New Orleans. Citizens Services was established in December 2004 to "assist persons and organizations who advance the interests of low- and moderate-income people," according to paperwork filed in Louisiana. In a 2006 ACORN publication, Citizen Services Inc. is described as "ACORN's campaign services entity."

Although located at the same address, CSI says they're a "separate organization entirely" despite the several links discussed in that article.

See also this group that tracks ACORN.

UPDATE 2: From this:

The Buckeye Institute, a Columbus-based think tank, today filed a state RICO action against the Association of Community Organizations for Reform Now (ACORN) on behalf of two Warren County voters. The action filed in Warren County Court of Common Pleas alleges ACORN has engaged in a pattern of corrupt activity that amounts to organized crime. It seeks ACORN's dissolution as a legal entity, the revocation of any licenses in Ohio, and an injunction against fraudulent voter registration and other illegal activities.

UPDATE 3: There's a long article about ACORN from 2003 here.

UPDATE 4: There's a BHO/ACORN timeline here.

UPDATE 5: There's a comprehensive page on ACORN here.