There are several companies supporting distributed advertising networks in Second Life. The distributed network usually means that company have partners, or publishers who display ads on their parcels.
The advertising materials (images along with landmarks, urls or notecards) are delivered to the publishers' parcels automatically.
SecondAds Grid-Wide Advertising
SecondAds is by far the largest pay-per-click advertisement network in Second Life with 10,000s of adboards located right accross the grid. They were also one of the very earliest to develop the technology and have since then invented a unique click fraud detection system that protects advertisers from fraud.
They have also developed the largest gaming network that is a unique tool for landowners to bring gaming traffic to their lands.
- SecondAds Advertisement Network
- Traffic Generation/Gaming Entertainment
SL Business Directory (SLBiz2Life)
SL Business Directory offers lot of second life advertising services, including:
- Free listing of SL business (the directory is seems to be a largest one at present time)
- Guaranteed Visitors
- Picks Booster (the tool to boost the business position in SL Seach)
- Second Life search analysis tools.
If you are looking for pay-per-click billboard ads, check their Ad Fusion network
- SL Business Directory (Second Life advertising services)
Subscribe-o-matic is a great collaboration tool which offers functionality similar to SL groups, but with extended features.
Apex is a SL vendor network which offers advertising services in form of web banners.
The large list of participating websites allows Apez to distribute your ad widely instead of displaying it on a single website.
http://www.apez.biz - Apez.promote advertising service
Check the advertising networks listing at official Second Life wiki page.