Financial services companies accounted for 28 percent of online display ads last month, up from 22 percent in September 2005 and 19 percent two years ago.

Financial services companies were the largest online advertiser category in September, followed by web media (19 percent of impressions), retail goods and services (15 percent), telecoms (9 percent), and public services (9 percent), writes MediaPost, citing new data from Nielsen/NetRatings AdRelevance.

Yahoo's email service accounted for the largest proportion of display ads - 42.4 percent. MySpace accounted for 15.4 percent, and MSN Hotmail 7.1 percent, according to Nielsen/NetRatings.

The overall number of cost-per-thousand impression display ads tracked totaled 232.1 billion; that's down a tad from 233.5 billion in August but up from 187.2 billion in July. AdRelevance's data doesn't account for online pre-roll ads, partnerships and sponsorships.