I would recommend Ultra VNC, which now supports Windows Vista and has file transfers.
if you're using Linux I'd highly recommend FreeNX. I've often found it a lot more responsive than VNC and it runs straight over SSH is secure and wouldn't require any additional ports.
I recommend NoMachine, they have both free and pay versions. Only for Linux do. Lot's of bells and whistles, and it's awesome.
But if you're on Linux and on a LAN, X forwarding is pretty awesome too (ssh -X myuser@server).
I recently stumbled over TeamViewer. Don't know how good it is compared to others really, but I like it a lot cause it Just Works. And cool thing is that it Just Works through routers and stuff like that without any setup.
Brilliant for doing remote support for friends and family for example =) Just make them run the client thing and give you the number and password. Voila.
Ultra VNC, mainly for its Single Click addon.
TightVNC has worked well for me.
RealVNC has always worked well for me. They have a free version but I broke down an bought the Person edition for some of the additional features it provides... like screen scaling.
We have moved all our users on multiple networks to UltraVNC. It offers file transfer, chat and is lightweight. It works quite well and is pretty fast. Compared to TightVNC and RealVNC which work fine, you get more bang for your buck with UltraVNC. I take issue with their web site, it's not that simple to navigate, but once you get the right version, it's all good.
Also, when in a pinch, we have used showmypc.com which temporarily installs TightVNC on a user's PC and let's you establish a connection without any initial install. It's free and it uses security codes to keep things on the up and up. Good luck!