Reverse auctions, where law firms submit successively lower bids on a matter or group of matters, are far from taking over the retention world (See my post of Aug. 9, 2011: huge overstatement by Ariba employee.). Even so, at least four groups offer online capabilities to enable the process.
ELawForum for years conducted sourcing bids (See my post of April 9, 2009: mentions at least 50 online bidding processes.) although I have heard that its founder, John Henry, has shifted or expanded to financing lawsuits.
Paul deJong (See my post of Jan. 20, 2007: LegalBenchmarket of Paul deJong.) runs sourcing processes like this primarily in Europe.
ProcureLaw (See my post of June 16, 2011: publicized savings from its use; and Aug. 15, 2011: overview of website.).
RFx Legal of Joe Freedman and Courtney Sapire enables online bidding for work and helps law departments locate appropriate firms.