I have used many financial supersites in order to track my portfolio and I find MSN Moneycentral to be the one that best meets my needs.
The one feature that stands out from the rest is its ability to allow you to enter and track each individual investment on an on-going basis. What I mean by this...I invest in several mutual funds for which monthly investments are made. Moneycentral allows you to record each individual component which will allow you to accurately calculate the gain/loss for each transaction as well as the average cost.
Moneycentral also allows you to export your portfolio to a tab delimited file. With this, it can be opened in Excel or any other spreadsheet program thereby allowing me to do plenty of "what-ifs".
Neither of these features are available with the other financial supersites (unless there is one that I haven't tried yet).
There are many other options available here as well as in the other supersites. I strongly encourage you to try them all....but I think you will find Moneycentral to be the best of the breed.

Recommended: Yes