Web-based personal finance management?

Not enough people use these packages. It's quite a lot of hassle for people to keep up with making the entries.

If I was needing personal finance software I would use Excel.

Although I suspect it is of little value to you, but if you were using Linux there is the KMymoney package which is kept up-to-date. There are other free Linux packages, the most widely known of which is probably Gnucash that is described as a double entry accounting programme.

There is also a proprietary package, Moneydance, which because it is based on Java is available for Linux, Windows, Mac OSX and Solaris. I haven't used it myself. I am not aware of any web-based "software on demand" type of package.

As Peter has said, one obvious solution for straightforward personal finance is to create your own spreadsheet. It should then be easily portable between computers and between operating systems.