iTeleport (if you need to tunnel over SSH)
Warning: Long post. This info is probably of more use to I.T. people and net-admins than those merely trying to share their home PC's desktop with the iPad. Still, I'm hoping some may find it useful.
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I thought I'd put in a plug for iTeleport since the OP asked specifically about VNC clients, (which I take to mean ones that use the actual VNC/RFB protocol, allowing you to connect to any compliant VNC server), not those that seem to use their own proprietary protocols such as Ignition or TeamViewer.
When I first set out to use my iPad for remote workstation/server administration, I had a specific requirement when choosing a VNC client app -- it had to support remote sessions through SSH tunnels. (By design, the VNC/RFB protocol doesn't include any form of sophisticated encryption natively -- this was viewed as being the responsibility of the transfer mechanism, not that of the core screen-sharing protocol)
What this means is that your remote system needs to accept SSH logins and all your VNC sessions are made through (and thus protected by) those SSH connections. You can configure a single host machine with both an SSH and VNC server, or more practically, set up a public-facing SSH server to allow TCP-forwarding for an entire internal network of VNC-enabled host machines. It also means that your firewall needs to be configured to allow incoming SSH logins -- this method does *not* rely on an external 3rd-party service to setup the connections the way some other remote desktop apps do.
Anyway, when I looked around, there were really only two candidates that supported VNC-over-SSH: iSSH and iTeleport. For SSH shell logins, iSSH is great. It allows you to do some nifty tricks with SSH tunnels and such. Since I already owned this app on my iPhone, it was was the first one I tried on my iPad as well. The problem is that the built-in VNC client is somewhat slow. Granted this can be attributed to a number of external factors and the iSSH author has been attempting to make performance improvements to this feature in the app. However, the fact is that my VNC sessions over it were pretty bad. So although I still consider iSSH to be invaluable for shell logins, it was pretty unusable (to me) as a VNC client.
That's when I found iTeleport. I wasn't real happy with the price ($29.95 US), but it was the only other app at the time that claimed to support SSH tunnels. After reading numerous reviews, I finally bought the app. It turns out that (with a few exceptions) it does everything I'd hoped and I now use it daily.
Currently, I use it to remote access three types of systems: the desktop consoles of VMs running on VMware Virtual Server on an Ubuntu host (Virtual Server, like Fusion on OSX has a built-in VNC server), WinXP workstations running UltraVNC and a couple of Macs using the native Screen Sharing feature with the VNC option enabled.
Besides the SSH tunneling capability, some notable features are the impressive performance of the screen-sharing itself, the ability to send just about any keystroke found on a standard 101-key PC keyboard and a decent mouse-pointer interface.
So what don't I like about the app? The price is a bit higher than I'd hoped. (That being said, it allows me to be much more productive, so I feel it was worth it. However, it is still on my list of the 5 most expensive apps I own) and updates aren't all that frequent. Yet, the single biggest problem I have with iTeleport is that it doesn't support public-key authentication for SSH connections, only passwords. This makes iTeleport unusable for a few systems I support. (iSSH supports pkeys with the ability to configure an arbitrary number of key pairs to any of its login profiles). With the release of iOS 4, it may be possible to use iTeleport in conjunction with iSSH to accomplish such a feat in a limited fashion, but it would really be nice to have all this available in a single tool.
Edit: Apparently Desktop Connect and the latest release of Jump Desktop supports SSH tunnels also. Anyone know if they do public-key authentication?