NetMeeting which is free, there are other
tools (but with costs involved) such as techinline.com, teamviewer.com,
None of those will allow logon to several PCs at once.
What do you mean by logon to the computers at once? You probably need to automate whatever you are doing when you need to logon to them more than find a way to logon to 30 computers at once. You still need to switch between them all to do anything anyway.
There is no logon tool that will log you on to several computers at once, you may be able to write a script that will fire off a remote connection with a pre-filled username/password, you can use vbscript and write in keystrokes and keypresses to fill in the info. So you'd type in your username, hit Tab, type in password, hit Tab, hit enter, and so on.
You can also create a new remote connection shorcut for each PC, save the credentials for them and then run them all at once.
Although I find it hard to think of a case where you would want to actually connect to that many computers at once without using tools like SMS or LANDesk to control the computers.