I've used a lot of them including VNC, TeamViewer, GoToMyPC, and Logmein.com. I've finally stuck with the free version of Logmein.com. TeamViewer is good but when I tried it someone had to be on the PC side to punch in the meeting code. Also, it cut me off after 10 mins after I used it a few times.
Logmein.com is very straight forward, works great, and only has a few limitations in the free version. The limitations are something like no sharing of sound, printers, and files (I think). I just use gmail to send myself a file if I need to or I send it through Skype (I have a separate Skype account on each computer).
I keep my old work PC on the net this way in case I need files. I also use this to maintain my in-laws PC. (Aren't we all the tech support guys?) -- working on their Mac upgrade now...