RDP file import: This feature allows you to load settings you have stored in RDP files. It can be quite useful if you already have many RDP files.

Navigation pane: Most of the RDP tools have a navigation pane on the left side where they list all your RDP connections in a hierarchical structure. The more RDP connections you have, the more useful is this feature.

Tabs: If you have opened multiple RDP connections, you can switch fast between servers by just clicking on the corresponding tab.

Task tray: This allows you to establish an RDP connection to a server by right clicking on the applications symbol in the task tray.

External Window: Some of the tools display the RDP connection only within their user interface. However, sometimes, it can be useful to connect to a TS server in an external window, for example when you are working with multiple monitors.

Window resizing: Only RoyalTS has this feature. They call it “Smart Size Mode” and it means that the desktop scales dynamically whenever you resize its window. So you won’t see scroll bars when you reduce the size of the RDP window, for example.

Centrally store credentials: This feature enables you to store the credentials for multiple RDP connections at a central location.

Credentials for groups: Some of the RDP tools allow you to assign credentials to groups of RDP definitions.

AD Scan: The Royal Phone Book extension for RoyalTS allows you to scan Active Directory to find your servers. I didn’t try this feature yet.

Custom RDP port: Usually RDP uses TCP port 3389. In some environments, it is necessary to change this, i.e. for security reasons.

Win2K8 features: Windows Server 2008 comes with a couple of new RDP features such as TS Gateway or Network Level Authentication. RD Tabs supports some of them.

ICA: If you have Citrix Metaframe servers to manage, you can use your RDP tool to connect to them.

VNC : VNC is a protocol that is popular in the Linux world. If you have to manage Linux servers, you will like this feature. Some use it for Windows server, too.

SSH: The same applies to SSH. If you manage your Linux servers without needing a graphical user interface, then you probably use SSH.

Telnet: I wonder if there are still admins using Telnet. But who knows, maybe security is not an issue in your organization.

See his results and findinds at this location:
http://4sysops.com/archives/comparison-of-six-free-rdp-client-tools/