DeadCr0w:

I have used a pretty good one called rAdmin .

Supports Windows NT security so you can give the right to remotely control, remotely view, telnet and/or transfer files to specific users or user groups. If the user is logged to a WinNT domain, Radmin Viewer uses his or her current username/password to authenticate.

Radmin uses a challenge-response password authentication method, based on 128 bit strong encryption—if Windows NT security support is switched off—to allow access to a remote computer.

128 bit strong encryption is used to encrypt all data streams and and it's optimized with a performance loss of only 5%.

Uses an IP filter so access to Radmin server is restricted to specified IP addresses and subnets.
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Free to try, $35 to buy.