Top Search Engine Marketing Agencies

if you are a mid to large sized retailer or publisher and are in need of a quality SEM, here are a few that top the list of decent search engine marketing agencies (in alphabetical order):

360i
avenue a | razorfish
iCrossing
iProspect
GSI - Global Strategies International
Performics

some smaller agencies that might be of interest:

BarkleyREI
Critical Mass
Zeta Interactive (formerly DigitalGrit)

i would definitely recommend choosing an agency that has experience in more than one area of online marketing as the synergies across programs could provide you with economies of scale. for example, having a single agency that manages your paid and natural search programs in parallel could save you a lot of money in the long run.