There is a kind of tax on savings and capital in the Netherlands but my understanding is that you won't have to pay it after leaving the country.
The way this works is that the tax office assumes a fictitious interest rate on all savings and you have to pay income tax on this. This is what's called “box 3 income“ on the income tax return. You don't have to report or pay taxes on the actual interest/rents/dividends you received. Currently, the tax rate is 30% on 4% interest, so 1.2% of your savings.
Furthermore, you do not pay any taxes on savings under a certain threshold (heffingsvrij vermogen), currently about €21000 if you are single. Dutch banks automatically give data on the amount of money on your bank accounts to the tax office.
Because the value of your savings on January 1st is what counts, those rules presumably do still apply to you for 2014. But I think that you cannot use the regular tax return application because you haven't lived in the Netherlands for the whole year. You can use the Mijn Belastingdienst website instead.
After that, if you live abroad, you do not have to pay taxes on all your savings but only on certain things like real estate property in the Netherlands. If you keep your bank account, I think the rules I described above would therefore not apply to you anymore. If you don't receive an invitation to submit a tax return from the tax office, you only have to submit one if you would pay more than €45 in taxes (you are then what's called buitenlands belastingplichtige).
I am not sure exactly how the 30% ruling plays into this. Apparently, you can chose to be considered “partially non-resident“ and therefore be exempted from income tax on savings but I am not sure whether you can do that after the fact.
Finally, if you have large savings, it might be worth seeking help from a professional. I did my best to double-check my answer but I am certainly not an expert.