Offensive Security is one the most active organization on the Internet about offsec. In addition to providing pentest and attack simulation services to their customers, they also bring a lot of great solution to the community.
Currently 6 online/live/in-house courses are available (pentest, wireless security, windows exploiation, web attacks…) who go with 5 different well known certifications. An incredible online virtual lab composed of intentionally vulnerable machine (ubuntu, XP, webapps, mail service, ftp vulns and so on…) has been created to enhance you security knowledge. Offensive Security is also editor of awesome projects such as Kali Linux, Metasploit and Google Hacking Database, the most used tools by pentesters.
The quality of all of these solutions is well recognized by the community and many companies and actually everyone who have to deals with offsec :)