See Duplicate Management for standard Salesforce Deduping. Use that unless you have specific needs. 

Other Apps

Duplicate Check for Salesforce

• https://appexchange.salesforce.com/listingDetail?listingId=a0N300000055fXxEAI 
• free for first 10,000 rows. 
• Batch and automated in paid plans
• Also merges up to 15 account. 
• May be good for a first cut bulk merge. 
• See pricing http://www.addressedworld.com/pricing/duplicate-check-for-salesforce
  • Reasonably priced, good features. 
  • EXCEPT, you can't dedupe custom objects without the Enterprise Plan for $1299 / year
  • Is number of records the number of records in your system available to be de-duped or the number of de-dupes performed? 

Dupe Blocker

• Free for NFP's so it is great! 
• Really good tool 

However

• You need to keep on top of the indexing - especially after a large import. 

Dupe Catcher

• Free - so until the SF one comes out then this one may be good enough - but push for a real dupe blocking tool like Ring Lead. 

Other Apps

Live Services

Work as you are editing the record. 

• Ring Lead http://www.ringlead.com/salesforce-applications/data-cleanse/
• Cleaner http://1771.co/portfolio-item/cleaner/

Cloud services

• Ring Lead http://www.ringlead.com/salesforce-applications/data-cleanse/
• Cloundingo

Bulk Services

• Advitya https://appexchange.salesforce.com/listingDetail?listingId=a0N30000000qqwZEAQ