They are just bots that crawl the forums all the time there is no need to worry about it,they are just checking for new content its when you dont get them you should start to worry.In the control panel you will find a section that allows all the major bots you can switch them all off but most people just leave them to do there work.
Ohh is it so...but i thought the msg says "Registered users" so maybe some users have registered in the forum as "Google Adsense [Bot]" and "Google [Bot]" and its that name...correct me if i am wrong pls..
No you are wrong the registered users:is the standard text that is always on php forum and if a member logs in you will see there name to the right of the registered user:if bots are just crawling they will be the only ones shown.If you go to control panel and look under general/spiders/robots you will see all the common spider and what time and date that they have visited the site.