Remote Assistance Program?

Hi Everyone
Im looking for a remote assistance software which is almost identical to Windows Remote Assistance.I’m in IT support and have looked at many but so far nothing is coming close to windows remote assistance.I want a user to be able to send a invitation via e-mail,and i dont want to have to install software on the clients pc.The connection must be established over the internet.I dont want to support any local (LAN) users,all are remote users belonging to different companies.Does anyone have any programs in mind???

Thanks in advance.